[SPARK-14511][BUILD] Upgrade genjavadoc to latest upstream
[SPARK-14511][BUILD] Upgrade genjavadoc to latest upstream

## What changes were proposed in this pull request?
In the past, genjavadoc had issues with package private members which led the spark project to use a forked version. This issue has been fixed upstream (typesafehub/genjavadoc#70) and a release is available for scala versions 2.10, 2.11 **and 2.12**, hence a forked version for spark is no longer necessary.
This pull request updates the build configuration to use the newest upstream genjavadoc.

## How was this patch tested?
The build was run `sbt unidoc`. During the process javadoc emits some errors on the generated java stubs, however these errors were also present before the upgrade. Furthermore, the produced html is fine.

Author: Jakob Odersky <jakob@odersky.com>

Closes #12707 from jodersky/SPARK-14511-genjavadoc.

big-pegasus/spark,holdenk/spark,UndeadBaneGitHub/spark,ahnqirage/spark,dbtsai/spark,janewangfb/spark,dbtsai/spark,holdenk/spark,jianran/spark,liyichao/spark,brad-kaiser/spark,aosagie/spark,adrian-ionescu/apache-spark,guoxiaolongzte/spark,mzl9039/spark,shubhamchopra/spark,nilsgrabbert/spark,loneknightpy/spark,kimoonkim/spark,wangmiao1981/spark,mdespriee/spark,maropu/spark,ddna1021/spark,1haodian/spark,nchammas/spark,facaiy/spark,wangmiao1981/spark,metamx/spark,rikima/spark,dbtsai/spark,darionyaphet/spark,ueshin/apache-spark,apache/spark,witgo/spark,milliman/spark,bravo-zhang/spark,ibm-research-ireland/sparkoscope,LantaoJin/spark,sureshthalamati/spark,ueshin/apache-spark,jrshust/spark,xuanyuanking/spark,dongjoon-hyun/spark,MLnick/spark,gengliangwang/spark,kimoonkim/spark,chuckchen/spark,skonto/spark,facaiy/spark,liutang123/spark,icexelloss/spark,loneknightpy/spark,wangyum/spark,joseph-torres/spark,big-pegasus/spark,witgo/spark,minixalpha/spark,ioana-delaney/spark,metamx/spark,JoshRosen/spark,publicRoman/spark,JerryLead/spark,stanzhai/spark,holdenk/spark,highfei2011/spark,guoxiaolongzte/spark,UndeadBaneGitHub/spark,shubhamchopra/spark,mahak/spark,ptkool/spark,techaddict/spark,BryanCutler/spark,milliman/spark,rezasafi/spark,nchammas/spark,ioana-delaney/spark,panfengfeng/spark,mzl9039/spark,srowen/spark,janewangfb/spark,esi-mineset/spark,someorz/spark,stanzhai/spark,kimoonkim/spark,wangmiao1981/spark,skonto/spark,WindCanDie/spark,WeichenXu123/spark,shubhamchopra/spark,esi-mineset/spark,taroplus/spark,adrian-ionescu/apache-spark,1haodian/spark,ajaysaini725/spark,patrick-nicholson/spark,ron8hu/spark,nilsgrabbert/spark,mahak/spark,darionyaphet/spark,andrewor14/spark,milliman/spark,xuanyuanking/spark,SnappyDataInc/spark,aosagie/spark,xuanyuanking/spark,jiangxb1987/spark,byakuinss/spark,ron8hu/spark,SnappyDataInc/spark,shaneknapp/spark,joseph-torres/spark,zhouyejoe/spark,andrewor14/spark,xflin/spark,caneGuy/spark,markhamstra/spark,dhruve/spark,jianran/spark,yanboliang/spark,rikima/spark,Panos-Bletsos/spark-cost-model-optimizer,jlopezmalla/spark,chuckchen/spark,taroplus/spark,narahari92/spark,jrshust/spark,aokolnychyi/spark,rednaxelafx/apache-spark,joseph-torres/spark,matthewfranglen/spark,sahilTakiar/spark,markhamstra/spark,bdrillard/spark,rednaxelafx/apache-spark,apache/spark,wangyum/spark,mike0sv/spark,Panos-Bletsos/spark-cost-model-optimizer,gioenn/xSpark,ibm-research-ireland/sparkoscope,aosagie/spark,patrick-nicholson/spark,shubhamchopra/spark,ajaysaini725/spark,guoxiaolongzte/spark,WindCanDie/spark,shuangshuangwang/spark,aray/spark,ahnqirage/spark,caneGuy/spark,sahilTakiar/spark,rednaxelafx/apache-spark,srowen/spark,adrian-ionescu/apache-spark,cloud-fan/spark,huang1900/spark,ron8hu/spark,xuanyuanking/spark,panfengfeng/spark,chuckchen/spark,vinodkc/spark,facaiy/spark,darionyaphet/spark,kiszk/spark,cin/spark,liutang123/spark,mike0sv/spark,shaneknapp/spark,ron8hu/spark,bdrillard/spark,MLnick/spark,wangyum/spark,darionyaphet/spark,1haodian/spark,highfei2011/spark,icexelloss/spark,zuotingbing/spark,guoxiaolongzte/spark,spark-test/spark,ajaysaini725/spark,xflin/spark,apache/spark,michalsenkyr/spark,markhamstra/spark,rezasafi/spark,witgo/spark,JoshRosen/spark,Aegeaner/spark,MLnick/spark,setjet/spark,zzcclp/spark,cloud-fan/spark,zzcclp/spark,facaiy/spark,dbtsai/spark,sureshthalamati/spark,goldmedal/spark,mike0sv/spark,vinodkc/spark,sahilTakiar/spark,guoxiaolongzte/spark,patrick-nicholson/spark,sachintyagi22/spark,big-pegasus/spark,liutang123/spark,ioana-delaney/spark,loneknightpy/spark,aokolnychyi/spark,lvdongr/spark,metamx/spark,rekhajoshm/spark,adrian-ionescu/apache-spark,techaddict/spark,markhamstra/spark,map222/spark,minixalpha/spark,publicRoman/spark,poffuomo/spark,goldmedal/spark,kiszk/spark,zhouyejoe/spark,wzhfy/spark,holdenk/spark,dongjoon-hyun/spark,ConeyLiu/spark,sahilTakiar/spark,jlopezmalla/spark,SHASHANKB/spark,ajaysaini725/spark,andrewor14/spark,zzcclp/spark,shaneknapp/spark,mzl9039/spark,maropu/spark,alunarbeach/spark,gioenn/xSpark,techaddict/spark,saltstar/spark,skonto/spark,holdenk/spark,JerryLead/spark,michalsenkyr/spark,SnappyDataInc/spark,srowen/spark,akopich/spark,taroplus/spark,goldmedal/spark,dhruve/spark,setjet/spark,zero323/spark,xuanyuanking/spark,cin/spark,huang1900/spark,zero323/spark,HyukjinKwon/spark,ericvandenbergfb/spark,dhruve/spark,rekhajoshm/spark,gioenn/xSpark,goldmedal/spark,mzl9039/spark,darionyaphet/spark,1haodian/spark,alunarbeach/spark,saltstar/spark,1haodian/spark,sachintyagi22/spark,pgandhi999/spark,aray/spark,ahnqirage/spark,saturday-shi/spark,cloud-fan/spark,guoxiaolongzte/spark,SHASHANKB/spark,JoshRosen/spark,jianran/spark,alunarbeach/spark,apache-spark-on-k8s/spark,jiangxb1987/spark,caneGuy/spark,ericvandenbergfb/spark,saturday-shi/spark,shaneknapp/spark,akopich/spark,ron8hu/spark,poffuomo/spark,esi-mineset/spark,kevinyu98/spark,ericvandenbergfb/spark,ptkool/spark,1haodian/spark,bOOm-X/spark,rikima/spark,eyalfa/spark,joseph-torres/spark,nlalevee/spark,HyukjinKwon/spark,akop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