Remove duplicated Channel ABC classes.

These were moved to IPython.kernel.channelsabc, but the old copies were
retained somehow.

Should we import them back into managerabc for backwards compatibility
if third party code has imported them from there? It seems unlikely that
they'd be used outside our codebase.
Merge pull request #5042 from takluyver/remove-channelabc-duplicates

Remove duplicated Channel ABC classes.