## What changes were proposed in this pull request?

According to the notice of the following Wiki front page, we can remove the obsolete wiki pointer safely in `README.md` and `docs/index.md`, too. These two lines are the last occurrence of that links.

```
All current wiki content has been merged into pages at http://spark.apache.org as of November 2016.
Each page links to the new location of its information on the Spark web site.
Obsolete wiki content is still hosted here, but carries a notice that it is no longer current.
```

## How was this patch tested?

Manual.

- `README.md`: https://github.com/dongjoon-hyun/spark/tree/remove_wiki_from_readme
- `docs/index.md`:
```
cd docs
SKIP_API=1 jekyll build
```
![screen shot 2016-12-09 at 2 53 29 pm](https://cloud.githubusercontent.com/assets/9700541/21067323/517252e2-be1f-11e6-85b1-2a4471131c5d.png)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #16239 from dongjoon-hyun/remove_wiki_from_readme.