Remove git_panel::GenerateCommitMessage in favor of git::GenerateCommitMessage (#33421)

`git_panel::GenerateCommitMessage` has no handler,
`git::GenerateCommitMessage` should be preferred. Could add a
`#[action(deprecated_aliases = ["git_panel::GenerateCommitMessage"])]`,
but decided not to because that action didn't work. So instead uses of
it will show up as keymap errors.

Closes #32667

Release Notes:

- N/A

crates/git_ui/src/git_panel.rs

@@ -83,7 +83,6 @@ actions!(
         FocusEditor,
         FocusChanges,
         ToggleFillCoAuthors,
-        GenerateCommitMessage
     ]
 );