Remove unused method to match up 4.1 branch

 Motivation:

4 and 5 were diverged long time ago and we recently reverted some of the
early commits in master.  We must make sure 4.1 and master are not very
different now.

Modification:

Remove ChannelHandlerInvokerUtil.invokeWriteAndFlush(...) as it is not used anyway and not exists in 4.1

Result:

4.1 and master got closer.