core: remove debugextrainmessage support
The stated purpose for this is for testing. However, it's not for testing of hg-git, but of git-remote-hg. The mode has been broken for years, though, and given that it doesn't actually rely on the known-good modes of hg-git, it doesn't offer much guarantee of anything actually working.
Removing as general cruft that needlessly complicates the codebase.
Closes: #281 (closed) !211 (closed)