refactor(vc): move git-commit to magit module

`git-commit` is no longer distributed with `magit` and no longer
declares its dependencies, causing "file missing: with-editor" and
similar errors for folks who don't have magit installed. Also, VC's
commit workflows don't utilize the mode, so there's no reason to keep it
in this module.

Ref: magit/magit@c170fcf399
Ref: #8003

modules/emacs/vc/config.el

@@ -142,27 +142,6 @@ info in the `header-line-format' is a more visible indicator."
         :n "gtc" #'git-timemachine-show-commit))
 
 
-(use-package! git-commit
-  :hook (doom-first-file . global-git-commit-mode)
-  :config
-  (set-yas-minor-mode! 'git-commit-mode)
-
-  ;; Enforce git commit conventions.
-  ;; See https://chris.beams.io/posts/git-commit/
-  (setq git-commit-summary-max-length 50
-        git-commit-style-convention-checks '(overlong-summary-line non-empty-second-line))
-  (setq-hook! 'git-commit-mode-hook fill-column 72)
-
-  (add-hook! 'git-commit-setup-hook
-    (defun +vc-start-in-insert-state-maybe-h ()
-      "Start git-commit-mode in insert state if in a blank commit message,
-otherwise in default state."
-      (when (and (bound-and-true-p evil-mode)
-                 (not (evil-emacs-state-p))
-                 (bobp) (eolp))
-        (evil-insert-state)))))
-
-
 (after! browse-at-remote
   ;; It's more sensible that the user have more options. If they want line
   ;; numbers, users can request them by making a selection first. Otherwise

modules/emacs/vc/packages.el

@@ -6,7 +6,6 @@
 (package! smerge-mode :built-in t)
 
 (package! browse-at-remote :pin "76aa27dfd469fcae75ed7031bb73830831aaccbf")
-(package! git-commit :pin "0aa26864e3fc4e6949711a4821caf6819e7ab171")
 (package! git-timemachine
   ;; The original lives on codeberg.org; which has uptime issues.
   :recipe (:host github :repo "emacsmirror/git-timemachine")