Remove conditional reference when targeting .NET 2.0
Remove conditional reference when targeting .NET 2.0

This fixed DevDiv.1158790 — Spurious errors, warnings and messages in IDE from background analysis, which
basically results in a bunch of warnings in the IDE:

     The referenced component 'C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETPortable\v4.5\Profile\Profile7\mscorlib.dll' could not be found.

C#/VB project system do not support conditional references and treat them as always being present. This <Reference was being included for all projects regardless
of the condition, confusing Visual Basic project system. Moved it to _ExplictReference concept which already exists for adding mscorlib (see <_ExplicitReference> in Microsoft.CSharp.CurrentVersionCommon.targets)

weltkante/roslyn,mgoertz-msft/roslyn,Giftednewt/roslyn,brettfo/roslyn,vslsnap/roslyn,taylorjonl/roslyn,dotnet/roslyn,mattwar/roslyn,jaredpar/roslyn,Hosch250/roslyn,zmaruo/roslyn,aanshibudhiraja/Roslyn,physhi/roslyn,supriyantomaftuh/roslyn,oberxon/roslyn,weltkante/roslyn,AmadeusW/roslyn,jkotas/roslyn,jonatassaraiva/roslyn,jasonmalinowski/roslyn,tmeschter/roslyn,yjfxfjch/roslyn,Pvlerick/roslyn,michalhosala/roslyn,Shiney/roslyn,khellang/roslyn,yeaicc/roslyn,jasonmalinowski/roslyn,VitalyTVA/roslyn,diryboy/roslyn,drognanar/roslyn,dpoeschl/roslyn,KiloBravoLima/roslyn,nemec/roslyn,MatthieuMEZIL/roslyn,CaptainHayashi/roslyn,oocx/roslyn,dpoeschl/roslyn,yjfxfjch/roslyn,FICTURE7/roslyn,jeremymeng/roslyn,moozzyk/roslyn,a-ctor/roslyn,lisong521/roslyn,jonatassaraiva/roslyn,MihaMarkic/roslyn-prank,AnthonyDGreen/roslyn,lisong521/roslyn,budcribar/roslyn,vcsjones/roslyn,DustinCampbell/roslyn,jroggeman/roslyn,managed-commons/roslyn,AdamSpeight2008/roslyn-AdamSpeight2008,pjmagee/roslyn,mseamari/Stuff,3F/roslyn,AnthonyDGreen/roslyn,leppie/roslyn,antiufo/roslyn,vslsnap/roslyn,abock/roslyn,AArnott/roslyn,taylorjonl/roslyn,reaction1989/roslyn,mattwar/roslyn,Hosch250/roslyn,ErikSchierboom/roslyn,srivatsn/roslyn,OmarTawfik/roslyn,KirillOsenkov/roslyn,devharis/roslyn,Pvlerick/roslyn,amcasey/roslyn,abock/roslyn,mseamari/Stuff,bbarry/roslyn,enginekit/roslyn,AdamSpeight2008/roslyn-AdamSpeight2008,paulvanbrenk/roslyn,MihaMarkic/roslyn-prank,AlexisArce/roslyn,FICTURE7/roslyn,OmarTawfik/roslyn,VPashkov/roslyn,HellBrick/roslyn,jhendrixMSFT/roslyn,oocx/roslyn,jhendrixMSFT/roslyn,budcribar/roslyn,mgoertz-msft/roslyn,jaredpar/roslyn,stephentoub/roslyn,ErikSchierboom/roslyn,khyperia/roslyn,jeremymeng/roslyn,jeffanders/roslyn,khyperia/roslyn,xoofx/roslyn,aanshibudhiraja/Roslyn,tmeschter/roslyn,kienct89/roslyn,MichalStrehovsky/roslyn,antonssonj/roslyn,shyamnamboodiripad/roslyn,jhendrixMSFT/roslyn,jkotas/roslyn,stephentoub/roslyn,zooba/roslyn,bartdesmet/roslyn,Inverness/roslyn,sharwell/roslyn,mirhagk/roslyn,KevinRansom/roslyn,davkean/roslyn,mirhagk/roslyn,xoofx/roslyn,AArnott/roslyn,ilyes14/roslyn,paulvanbrenk/roslyn,enginekit/roslyn,v-codeel/roslyn,HellBrick/roslyn,AlekseyTs/roslyn,MattWindsor91/roslyn,budcribar/roslyn,stebet/roslyn,SeriaWei/roslyn,drognanar/roslyn,natgla/roslyn,MihaMarkic/roslyn-prank,stebet/roslyn,moozzyk/roslyn,KevinH-MS/roslyn,srivatsn/roslyn,orthoxerox/roslyn,mavasani/roslyn,managed-commons/roslyn,VSadov/roslyn,balajikris/roslyn,dotnet/roslyn,Pvlerick/roslyn,CaptainHayashi/roslyn,leppie/roslyn,thomaslevesque/roslyn,KirillOsenkov/roslyn,physhi/roslyn,evilc0des/roslyn,agocke/roslyn,AmadeusW/roslyn,oocx/roslyn,pjmagee/roslyn,mattscheffer/roslyn,paulvanbrenk/roslyn,reaction1989/roslyn,KirillOsenkov/roslyn,pjmagee/roslyn,AnthonyDGreen/roslyn,stebet/roslyn,akrisiun/roslyn,antonssonj/roslyn,gafter/roslyn,Inverness/roslyn,swaroop-sridhar/roslyn,mgoertz-msft/roslyn,jamesqo/roslyn,v-codeel/roslyn,diryboy/roslyn,thomaslevesque/roslyn,oberxon/roslyn,akrisiun/roslyn,jroggeman/roslyn,ilyes14/roslyn,jcouv/roslyn,natidea/roslyn,tvand7093/roslyn,yeaicc/roslyn,AlexisArce/roslyn,tannergooding/roslyn,russpowers/roslyn,danielcweber/roslyn,VSadov/roslyn,GuilhermeSa/roslyn,weltkante/roslyn,michalhosala/roslyn,mmitche/roslyn,brettfo/roslyn,davkean/roslyn,pdelvo/roslyn,stephentoub/roslyn,wvdd007/roslyn,mirhagk/roslyn,natgla/roslyn,aelij/roslyn,dpoeschl/roslyn,robinsedlaczek/roslyn,GuilhermeSa/roslyn,MichalStrehovsky/roslyn,jmarolf/roslyn,russpowers/roslyn,Maxwe11/roslyn,evilc0des/roslyn,bbarry/roslyn,devharis/roslyn,heejaechang/roslyn,KiloBravoLima/roslyn,CaptainHayashi/roslyn,Maxwe11/roslyn,tmeschter/roslyn,xasx/roslyn,danielcweber/roslyn,genlu/roslyn,VShangxiao/roslyn,ericfe-ms/roslyn,SeriaWei/roslyn,dovzhikova/roslyn,tmat/roslyn,rgani/roslyn,robinsedlaczek/roslyn,mmitche/roslyn,3F/roslyn,KevinRansom/roslyn,magicbing/roslyn,amcasey/roslyn,natidea/roslyn,KevinH-MS/roslyn,jmarolf/roslyn,sharadagrawal/Roslyn,YOTOV-LIMITED/roslyn,natgla/roslyn,agocke/roslyn,grianggrai/roslyn,nagyistoce/roslyn,akrisiun/roslyn,cston/roslyn,abock/roslyn,bkoelman/roslyn,jkotas/roslyn,vcsjones/roslyn,TyOverby/roslyn,krishnarajbb/roslyn,wvdd007/roslyn,KiloBravoLima/roslyn,zooba/roslyn,tmat/roslyn,jamesqo/roslyn,VPashkov/roslyn,lisong521/roslyn,jmarolf/roslyn,enginekit/roslyn,shyamnamboodiripad/roslyn,kelltrick/roslyn,balajikris/roslyn,tang7526/roslyn,supriyantomaftuh/roslyn,managed-commons/roslyn,jasonmalinowski/roslyn,KamalRathnayake/roslyn,tvand7093/roslyn,ahmedshuhel/roslyn,ValentinRueda/roslyn,MichalStrehovsky/roslyn,MatthieuMEZIL/roslyn,MattWindsor91/roslyn,tmat/roslyn,jbhensley/roslyn,a-ctor/roslyn,antonssonj/roslyn,xasx/roslyn,TyOverby/roslyn,VShangxiao/roslyn,grianggrai/roslyn,krishnarajbb/roslyn,Shiney/roslyn,ValentinRueda/roslyn,michalhosala/roslyn,huoxudong125/roslyn,ljw1004/roslyn,doconnell565/roslyn,tang7526/roslyn,heejaechang/roslyn,vcsjones/roslyn,robinsedlaczek/roslyn,devharis/roslyn,panopticoncentral/roslyn,kienct89/roslyn,supriyantomaftuh/roslyn,jamesqo/roslyn,pdelvo/roslyn,CyrusNajmabadi/roslyn,RipCurrent/roslyn,bartdesmet/roslyn,ahmedshuhel/roslyn,VShangxiao/roslyn,MattWindsor91/roslyn,MatthieuMEZIL/roslyn,thomaslevesque/roslyn,genlu/roslyn,DustinCampbell/roslyn,Inverness/roslyn,dotnet/roslyn,bkoelman/roslyn,KevinRansom/roslyn,ljw1004/roslyn,ericfe-ms/roslyn,balajikris/roslyn,huoxudong125/roslyn,bbarry/roslyn,ilyes14/roslyn,jroggeman/roslyn,mseamari/Stuff,orthoxerox/roslyn,mattscheffer/roslyn,tannergooding/roslyn,AArnott/roslyn,jcouv/roslyn,jaredpar/roslyn,EricArndt/roslyn,davkean/roslyn,jeffanders/roslyn,taylorjonl/roslyn,zmaruo/roslyn,aelij/roslyn,basoundr/roslyn,Hosch250/roslyn,mavasani/roslyn,magicbing/roslyn,bartdesmet/roslyn,rgani/roslyn,basoundr/roslyn,aelij/roslyn,YOTOV-LIMITED/roslyn,3F/roslyn,VPashkov/roslyn,nguerrera/roslyn,eriawan/roslyn,mmitche/roslyn,grianggrai/roslyn,bkoelman/roslyn,poizan42/roslyn,ValentinRueda/roslyn,jeffanders/roslyn,jeremymeng/roslyn,khellang/roslyn,brettfo/roslyn,ljw1004/roslyn,ErikSchierboom/roslyn,leppie/roslyn,ahmedshuhel/roslyn,MattWindsor91/roslyn,eriawan/roslyn,heejaechang/roslyn,drognanar/roslyn,eriawan/roslyn,VitalyTVA/roslyn,xasx/roslyn,sharadagrawal/Roslyn,v-codeel/roslyn,RipCurrent/roslyn,dovzhikova/roslyn,amcasey/roslyn,KamalRathnayake/roslyn,panopticoncentral/roslyn,magicbing/roslyn,nguerrera/roslyn,tang7526/roslyn,EricArndt/roslyn,mattscheffer/roslyn,kelltrick/roslyn,VitalyTVA/roslyn,Giftednewt/roslyn,Maxwe11/roslyn,basoundr/roslyn,oberxon/roslyn,antiufo/roslyn,pdelvo/roslyn,natidea/roslyn,GuilhermeSa/roslyn,wvdd007/roslyn,AmadeusW/roslyn,jbhensley/roslyn,RipCurrent/roslyn,srivatsn/roslyn,zmaruo/roslyn,xoofx/roslyn,zooba/roslyn,FICTURE7/roslyn,krishnarajbb/roslyn,OmarTawfik/roslyn,Giftednewt/roslyn,shyamnamboodiripad/roslyn,nagyistoce/roslyn,lorcanmooney/roslyn,yeaicc/roslyn,rgani/roslyn,mattwar/roslyn,antiufo/roslyn,yjfxfjch/roslyn,doconnell565/roslyn,AdamSpeight2008/roslyn-AdamSpeight2008,lorcanmooney/roslyn,reaction1989/roslyn,TyOverby/roslyn,jonatassaraiva/roslyn,orthoxerox/roslyn,jcouv/roslyn,evilc0des/roslyn,ericfe-ms/roslyn,cston/roslyn,nagyistoce/roslyn,doconnell565/roslyn,vslsnap/roslyn,VSadov/roslyn,gafter/roslyn,nemec/roslyn,KamalRathnayake/roslyn,nemec/roslyn,russpowers/roslyn,kelltrick/roslyn,a-ctor/roslyn,agocke/roslyn,KevinH-MS/roslyn,EricArndt/roslyn,CyrusNajmabadi/roslyn,CyrusNajmabadi/roslyn,sharadagrawal/Roslyn,jbhensley/roslyn,SeriaWei/roslyn,physhi/roslyn,mavasani/roslyn,khellang/roslyn,diryboy/roslyn,panopticoncentral/roslyn,Shiney/roslyn,danielcweber/roslyn,tvand7093/roslyn,AlekseyTs/roslyn,cston/roslyn,swaroop-sridhar/roslyn,poizan42/roslyn,YOTOV-LIMITED/roslyn,AlexisArce/roslyn,sharwell/roslyn,AlekseyTs/roslyn,dovzhikova/roslyn,moozzyk/roslyn,huoxudong125/roslyn,poizan42/roslyn,lorcanmooney/roslyn,tannergooding/roslyn,sharwell/roslyn,swaroop-sridhar/roslyn,nguerrera/roslyn,aanshibudhiraja/Roslyn,khyperia/roslyn,DustinCampbell/roslyn,genlu/roslyn,HellBrick/roslyn,gafter/roslyn,kienct89/roslyn