Remove Mercurial support for Bitbucket.
Review Request #1961 — Created July 13, 2020 and updated
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. Testing Done: Unit tests pass. Reviewed at https://reviews.reviewboard.org/r/11041/