Remove language::markdown (#25136)

The language::markdown crate had been superceded by markdown::Mardown.

After #25117, the only two remaining use-cases were rendering git commit
messages (which are arguably not really markdown) and the signature help
(which is definitely not markdown).

Updated the former to use the new markdown component, and the latter to
do syntax highlighting manually.

Release Notes:

- Allow selecting the commit message in git commits