[SPARK-18744][CORE] Remove workaround for Netty memory leak

## What changes were proposed in this pull request?

We added some codes in https://github.com/apache/spark/pull/14961 because of https://github.com/netty/netty/issues/5833

Now we can remove them as it's fixed in Netty 4.0.42.Final.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16167 from zsxwing/remove-netty-workaround.