Remove git_commit syntax highlighting from core Zed (#9025)

Fallback to extension
https://github.com/zed-industries/extensions/pull/307

Release Notes:

- Remove git_commit syntax highlighting from Zed core, `git-firefly` extension replaced that

Co-authored-by: William Desportes <williamdes@wdes.fr>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

crates/languages/src/gitcommit/injections.scm

@@ -1,8 +0,0 @@
-((diff) @injection.content
- (#set! injection.combined)
- (#set! injection.language "diff"))
-
-((rebase_command) @injection.content
- (#set! injection.combined)
- (#set! injection.language "git_rebase"))
-