Revert "[#1058] Fix incorrect logic to detect if ChannelHandlerContext is writable"

This reverts commit 3acb22c38c7c37600827e8d9a478bce9b5f6432a.
Revert "[#1058] Add Channel/ChannelHandlerContext/ChannelPipeline.isWritable()"

This reverts commit 4e36fbca58138447dab10d69933b1633938640b3.