[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.