[SPARK-13874][DOC] Remove docs of streaming-akka, streaming-zeromq,...

[SPARK-13874][DOC] Remove docs of streaming-akka, streaming-zeromq, streaming-mqtt and streaming-twitter

## What changes were proposed in this pull request?

This PR removes all docs about the old streaming-akka, streaming-zeromq, streaming-mqtt and streaming-twitter projects since I have already copied them to https://github.com/spark-packages

Also remove mqtt_wordcount.py that I forgot to remove previously.

## How was this patch tested?

Jenkins PR Build.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11824 from zsxwing/remove-doc.