[MINOR][DOCS] Remove Apache Spark Wiki address

## 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Obsolete wiki content is still hosted here, but carries a notice that it is no longer current.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 apache#16239 from dongjoon-hyun/remove_wiki_from_readme.

README.md

@@ -13,8 +13,7 @@ and Spark Streaming for stream processing.
 ## Online Documentation
 
 You can find the latest Spark documentation, including a programming
-guide, on the [project web page](http://spark.apache.org/documentation.html)
-and [project wiki](https://cwiki.apache.org/confluence/display/SPARK).
+guide, on the [project web page](http://spark.apache.org/documentation.html).
 This README file only contains basic setup instructions.
 
 ## Building Spark

docs/index.md

@@ -131,7 +131,6 @@ options for deployment:
 **External Resources:**
 
 * [Spark Homepage](http://spark.apache.org)
-* [Spark Wiki](https://cwiki.apache.org/confluence/display/SPARK)
 * [Spark Community](http://spark.apache.org/community.html) resources, including local meetups
 * [StackOverflow tag `apache-spark`](http://stackoverflow.com/questions/tagged/apache-spark)
 * [Mailing Lists](http://spark.apache.org/mailing-lists.html): ask questions about Spark here