Remove 'RoslynCompilerExtensionActive' variable
Remove 'RoslynCompilerExtensionActive' variable

When the CompilerPackage was installed into multiple hives, the current
hives  .props file would set the "RoslynCompilerExtensionActive"
variable to true, and then ALL hives .targets files would observe that
true value, leading to incorrect values for CscToolPath and other
variables.

This change removes the RoslynCompilerExtensionActive variable
completely, and inlines the actual hive check everywhere it was used.

genlu/roslyn,dotnet/roslyn,abock/roslyn,Giftednewt/roslyn,jhendrixMSFT/roslyn,jasonmalinowski/roslyn,mgoertz-msft/roslyn,shyamnamboodiripad/roslyn,jmarolf/roslyn,davkean/roslyn,ljw1004/roslyn,davkean/roslyn,budcribar/roslyn,jhendrixMSFT/roslyn,jamesqo/roslyn,MatthieuMEZIL/roslyn,KevinH-MS/roslyn,AlekseyTs/roslyn,a-ctor/roslyn,mavasani/roslyn,rgani/roslyn,KirillOsenkov/roslyn,tmeschter/roslyn,sharwell/roslyn,KevinH-MS/roslyn,jkotas/roslyn,xoofx/roslyn,gafter/roslyn,brettfo/roslyn,paulvanbrenk/roslyn,vslsnap/roslyn,sharadagrawal/Roslyn,agocke/roslyn,ljw1004/roslyn,drognanar/roslyn,mattwar/roslyn,stephentoub/roslyn,OmarTawfik/roslyn,wvdd007/roslyn,basoundr/roslyn,pdelvo/roslyn,mavasani/roslyn,sharadagrawal/Roslyn,TyOverby/roslyn,swaroop-sridhar/roslyn,bkoelman/roslyn,physhi/roslyn,mmitche/roslyn,jaredpar/roslyn,mmitche/roslyn,stephentoub/roslyn,KevinH-MS/roslyn,MichalStrehovsky/roslyn,zooba/roslyn,bartdesmet/roslyn,mmitche/roslyn,KiloBravoLima/roslyn,orthoxerox/roslyn,natidea/roslyn,KiloBravoLima/roslyn,jeffanders/roslyn,akrisiun/roslyn,brettfo/roslyn,kelltrick/roslyn,shyamnamboodiripad/roslyn,AmadeusW/roslyn,eriawan/roslyn,CyrusNajmabadi/roslyn,tannergooding/roslyn,dpoeschl/roslyn,michalhosala/roslyn,mattscheffer/roslyn,jamesqo/roslyn,AdamSpeight2008/roslyn-AdamSpeight2008,orthoxerox/roslyn,diryboy/roslyn,nguerrera/roslyn,mgoertz-msft/roslyn,gafter/roslyn,pdelvo/roslyn,Hosch250/roslyn,tannergooding/roslyn,balajikris/roslyn,agocke/roslyn,cston/roslyn,ErikSchierboom/roslyn,nguerrera/roslyn,MattWindsor91/roslyn,DustinCampbell/roslyn,tmat/roslyn,brettfo/roslyn,bartdesmet/roslyn,robinsedlaczek/roslyn,KiloBravoLima/roslyn,AArnott/roslyn,ericfe-ms/roslyn,KevinRansom/roslyn,abock/roslyn,MichalStrehovsky/roslyn,jeffanders/roslyn,kelltrick/roslyn,eriawan/roslyn,DustinCampbell/roslyn,panopticoncentral/roslyn,cston/roslyn,basoundr/roslyn,leppie/roslyn,bkoelman/roslyn,lorcanmooney/roslyn,jasonmalinowski/roslyn,nguerrera/roslyn,tmeschter/roslyn,genlu/roslyn,rgani/roslyn,KevinRansom/roslyn,jasonmalinowski/roslyn,AArnott/roslyn,gafter/roslyn,dpoeschl/roslyn,rgani/roslyn,mattwar/roslyn,khyperia/roslyn,leppie/roslyn,jkotas/roslyn,aelij/roslyn,sharwell/roslyn,amcasey/roslyn,aelij/roslyn,xoofx/roslyn,reaction1989/roslyn,AnthonyDGreen/roslyn,kelltrick/roslyn,robinsedlaczek/roslyn,Giftednewt/roslyn,MatthieuMEZIL/roslyn,panopticoncentral/roslyn,bartdesmet/roslyn,amcasey/roslyn,DustinCampbell/roslyn,Pvlerick/roslyn,CaptainHayashi/roslyn,physhi/roslyn,vslsnap/roslyn,natidea/roslyn,mattscheffer/roslyn,khyperia/roslyn,yeaicc/roslyn,reaction1989/roslyn,stephentoub/roslyn,CaptainHayashi/roslyn,VSadov/roslyn,dotnet/roslyn,balajikris/roslyn,jcouv/roslyn,lorcanmooney/roslyn,Shiney/roslyn,TyOverby/roslyn,VSadov/roslyn,balajikris/roslyn,bkoelman/roslyn,mavasani/roslyn,shyamnamboodiripad/roslyn,dotnet/roslyn,khellang/roslyn,Pvlerick/roslyn,jaredpar/roslyn,khellang/roslyn,Hosch250/roslyn,heejaechang/roslyn,robinsedlaczek/roslyn,akrisiun/roslyn,leppie/roslyn,diryboy/roslyn,bbarry/roslyn,khyperia/roslyn,jamesqo/roslyn,xasx/roslyn,OmarTawfik/roslyn,eriawan/roslyn,ericfe-ms/roslyn,paulvanbrenk/roslyn,AnthonyDGreen/roslyn,CyrusNajmabadi/roslyn,ericfe-ms/roslyn,zooba/roslyn,reaction1989/roslyn,aelij/roslyn,CyrusNajmabadi/roslyn,tannergooding/roslyn,tmat/roslyn,KevinRansom/roslyn,a-ctor/roslyn,Giftednewt/roslyn,diryboy/roslyn,srivatsn/roslyn,dpoeschl/roslyn,abock/roslyn,tvand7093/roslyn,yeaicc/roslyn,AlekseyTs/roslyn,wvdd007/roslyn,KirillOsenkov/roslyn,orthoxerox/roslyn,ErikSchierboom/roslyn,tmeschter/roslyn,MatthieuMEZIL/roslyn,jhendrixMSFT/roslyn,swaroop-sridhar/roslyn,heejaechang/roslyn,wvdd007/roslyn,sharadagrawal/Roslyn,drognanar/roslyn,michalhosala/roslyn,budcribar/roslyn,MattWindsor91/roslyn,ljw1004/roslyn,basoundr/roslyn,a-ctor/roslyn,mgoertz-msft/roslyn,amcasey/roslyn,Hosch250/roslyn,MichalStrehovsky/roslyn,xasx/roslyn,jmarolf/roslyn,Shiney/roslyn,jcouv/roslyn,mattwar/roslyn,KirillOsenkov/roslyn,OmarTawfik/roslyn,AArnott/roslyn,jcouv/roslyn,lorcanmooney/roslyn,genlu/roslyn,xoofx/roslyn,AlekseyTs/roslyn,paulvanbrenk/roslyn,weltkante/roslyn,pdelvo/roslyn,khellang/roslyn,natidea/roslyn,CaptainHayashi/roslyn,swaroop-sridhar/roslyn,jmarolf/roslyn,jkotas/roslyn,Pvlerick/roslyn,zooba/roslyn,panopticoncentral/roslyn,tvand7093/roslyn,jeffanders/roslyn,TyOverby/roslyn,budcribar/roslyn,cston/roslyn,agocke/roslyn,AnthonyDGreen/roslyn,Shiney/roslyn,ErikSchierboom/roslyn,MattWindsor91/roslyn,physhi/roslyn,xasx/roslyn,bbarry/roslyn,davkean/roslyn,weltkante/roslyn,srivatsn/roslyn,michalhosala/roslyn,AdamSpeight2008/roslyn-AdamSpeight2008,tvand7093/roslyn,drognanar/roslyn,srivatsn/roslyn,vslsnap/roslyn,akrisiun/roslyn,tmat/roslyn,AmadeusW/roslyn,bbarry/roslyn,AdamSpeight2008/roslyn-AdamSpeight2008,weltkante/roslyn,VSadov/roslyn,AmadeusW/roslyn,mattscheffer/roslyn,MattWindsor91/roslyn,yeaicc/roslyn,heejaechang/roslyn,sharwell/roslyn,jaredpar/roslyn