Remove Mercurial support for Bitbucket.
Review Request #11041 — Created June 10, 2020 and submitted
Bitbucket has sadly removed their Mercurial support, so we're doing the
same with our Bitbucket support.
Older repositoriy configurations in Review Board will still remain, but
of course are no longer accessible. We disallow configuring new
Mercurial-based repositories, though, leaving in only Git.
Unit tests pass.