Remove old code that is not needed anymore with newer Netty versions.

This code also produced a deadlock with 3.6.0.Final and later as the
VirtualExecutorService.awaitTermination(..) has a bug which is triggered
by the new shutdown logic which was introduced with 3.6.0.Final.
Merge pull request #760 from normanmaurer/shutdown_hang

Remove old code that is not needed anymore with newer Netty versions.