Motivation:

If ChannelPipeline.removeHandlers is internal it is extremely difficult
to implement a correct ChannelCore. For that reason, we should make it
available as an extension on ChannelCore.

This is a minor layering violation, but that's ok: ChannelCore and
Channel are not actually intended to be separate objects in most cases,
they are just separate for code clarity reasons.

Modifications:

Add ChannelCore.removeHandlers as a public method.

Result:

Easier to implement ChannelCore.