[SPARK-13888][DOC] Remove Akka Receiver doc and refer to the DStream Akka project

## What changes were proposed in this pull request?

I have copied the docs of Streaming Akka to https://github.com/spark-packages/dstream-akka/blob/master/README.md

So we can remove them from Spark now.

## How was this patch tested?

Only document changes.

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11711 from zsxwing/remove-akka-doc.