[SPARK-13942][CORE][DOCS] Remove Shark-related docs for 2.x
[SPARK-13942][CORE][DOCS] Remove Shark-related docs for 2.x

## What changes were proposed in this pull request?

`Shark` was merged into `Spark SQL` since [July 2014](https://databricks.com/blog/2014/07/01/shark-spark-sql-hive-on-spark-and-the-future-of-sql-on-spark.html). The followings seem to be the only legacy. For Spark 2.x, we had better clean up those docs.

**Migration Guide**
```
- ## Migration Guide for Shark Users
- ...
- ### Scheduling
- ...
- ### Reducer number
- ...
- ### Caching
```

## How was this patch tested?

Pass the Jenkins test.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11770 from dongjoon-hyun/SPARK-13942.

ConeyLiu/spark,tengpeng/spark,SnappyDataInc/spark,narahari92/spark,WeichenXu123/spark,apache/spark,jianran/spark,debugger87/spark,shaneknapp/spark,LantaoJin/spark,sahilTakiar/spark,pgandhi999/spark,minixalpha/spark,someorz/spark,vinodkc/spark,facaiy/spark,apache-spark-on-k8s/spark,ptkool/spark,ahnqirage/spark,facaiy/spark,liutang123/spark,maropu/spark,publicRoman/spark,jrshust/spark,kissmetrics/spark,wangmiao1981/spark,joseph-torres/spark,shaneknapp/spark,holdenk/spark,narahari92/spark,jrshust/spark,ueshin/apache-spark,mzl9039/spark,esi-mineset/spark,zero323/spark,caneGuy/spark,aray/spark,ioana-delaney/spark,jianran/spark,rikima/spark,jrshust/spark,minixalpha/spark,kevinyu98/spark,xflin/spark,bravo-zhang/spark,stanzhai/spark,minixalpha/spark,techaddict/spark,skonto/spark,kevinyu98/spark,liyichao/spark,aray/spark,JerryLead/spark,Aegeaner/spark,milliman/spark,shaneknapp/spark,aokolnychyi/spark,nchammas/spark,poffuomo/spark,sachintyagi22/spark,sureshthalamati/spark,facaiy/spark,ahnqirage/spark,jrshust/spark,WindCanDie/spark,aosagie/spark,caneGuy/spark,taroplus/spark,WeichenXu123/spark,zuotingbing/spark,kimoonkim/spark,chuckchen/spark,guoxiaolongzte/spark,cloud-fan/spark,tejasapatil/spark,apache/spark,Panos-Bletsos/spark-cost-model-optimizer,rezasafi/spark,skonto/spark,taroplus/spark,darionyaphet/spark,jianran/spark,facaiy/spark,eyalfa/spark,mdespriee/spark,wangyum/spark,saturday-shi/spark,1haodian/spark,saturday-shi/spark,ConeyLiu/spark,saltstar/spark,ibm-research-ireland/sparkoscope,nilsgrabbert/spark,JoshRosen/spark,kissmetrics/spark,Aegeaner/spark,cin/spark,ericvandenbergfb/spark,WindCanDie/spark,maropu/spark,guoxiaolongzte/spark,aokolnychyi/spark,joseph-torres/spark,liyichao/spark,nchammas/spark,ajaysaini725/spark,pgandhi999/spark,guoxiaolongzte/spark,srowen/spark,janewangfb/spark,shubhamchopra/spark,bOOm-X/spark,eyalfa/spark,saltstar/spark,BryanCutler/spark,ibm-research-ireland/sparkoscope,dbtsai/spark,goldmedal/spark,publicRoman/spark,bravo-zhang/spark,ibm-research-ireland/sparkoscope,eyalfa/spark,panfengfeng/spark,holdenk/spark,nilsgrabbert/spark,lvdongr/spark,MLnick/spark,jrshust/spark,kissmetrics/spark,kimoonkim/spark,skonto/spark,BryanCutler/spark,zhouyejoe/spark,vinodkc/spark,publicRoman/spark,maropu/spark,lvdongr/spark,JoshRosen/spark,actuaryzhang/spark,rikima/spark,BryanCutler/spark,jlopezmalla/spark,wangmiao1981/spark,apache-spark-on-k8s/spark,nilsgrabbert/spark,bdrillard/spark,stanzhai/spark,michalsenkyr/spark,WindCanDie/spark,rezasafi/spark,zzcclp/spark,stanzhai/spark,jkbradley/spark,saltstar/spark,JerryLead/spark,actuaryzhang/spark,janewangfb/spark,hvanhovell/spark,sahilTakiar/spark,Panos-Bletsos/spark-cost-model-optimizer,witgo/spark,szhem/spark,ajaysaini725/spark,bravo-zhang/spark,ericvandenbergfb/spark,SnappyDataInc/spark,szhem/spark,adrian-ionescu/apache-spark,gioenn/xSpark,HyukjinKwon/spark,aokolnychyi/spark,wangmiao1981/spark,janewangfb/spark,zzcclp/spark,LantaoJin/spark,adrian-ionescu/apache-spark,patrick-nicholson/spark,rekhajoshm/spark,zhouyejoe/spark,1haodian/spark,szhem/spark,wangmiao1981/spark,apache-spark-on-k8s/spark,MLnick/spark,goldmedal/spark,kiszk/spark,jiangxb1987/spark,bdrillard/spark,zuotingbing/spark,rednaxelafx/apache-spark,wangyum/spark,aokolnychyi/spark,metamx/spark,andrewor14/spark,cloud-fan/spark,lxsmnv/spark,hvanhovell/spark,liyichao/spark,milliman/spark,bravo-zhang/spark,metamx/spark,panfengfeng/spark,highfei2011/spark,ioana-delaney/spark,apache/spark,1haodian/spark,dbtsai/spark,patrick-nicholson/spark,icexelloss/spark,sachintyagi22/spark,ron8hu/spark,rekhajoshm/spark,icexelloss/spark,debugger87/spark,jlopezmalla/spark,kimoonkim/spark,ioana-delaney/spark,saltstar/spark,JoshRosen/spark,jkbradley/spark,someorz/spark,Aegeaner/spark,kissmetrics/spark,HyukjinKwon/spark,lvdongr/spark,huang1900/spark,joseph-torres/spark,kimoonkim/spark,andrewor14/spark,WeichenXu123/spark,zhouyejoe/spark,saltstar/spark,ahnqirage/spark,rednaxelafx/apache-spark,xflin/spark,stanzhai/spark,map222/spark,metamx/spark,tengpeng/spark,hhbyyh/spark,panfengfeng/spark,caneGuy/spark,rednaxelafx/apache-spark,szhem/spark,xflin/spark,chuckchen/spark,loneknightpy/spark,setjet/spark,Aegeaner/spark,patrick-nicholson/spark,chuckchen/spark,MLnick/spark,ericvandenbergfb/spark,rezasafi/spark,huang1900/spark,shuangshuangwang/spark,spark-test/spark,aray/spark,map222/spark,mike0sv/spark,huang1900/spark,byakuinss/spark,dongjoon-hyun/spark,akopich/spark,witgo/spark,vinodkc/spark,byakuinss/spark,goldmedal/spark,ron8hu/spark,dhruve/spark,xuanyuanking/spark,map222/spark,andrewor14/spark,lxsmnv/spark,JerryLead/spark,lxsmnv/spark,big-pegasus/spark,JoshRosen/spark,zero323/spark,byakuinss/spark,sureshthalamati/spark,stanzhai/spark,alunarbeach/spark,tengpeng/spark,Panos-Bletsos/spark-cost-model-optimizer,aosagie/spark,adrian-ionescu/apache-spark,publicRoman/spark,metamx/spark,pgandhi999/spark,mzl9039/spark,chuckchen/spark,markhamstra/spark,patrick-nicholson/spark,LantaoJin/spark,stanzhai/spark,kiszk/spark,map222/spark,ron8hu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