Remove undocumented support of signing key in the repository git configuration file (#36143)

Per-repository signing keys have never been officially supported, as
they would require users to modify the repository’s config file.

At this point, it is clear that only global signing keys (GPG or SSH)
should be allowed. If we want to introduce per-repository signing keys
in the future, it will require a complete design proposal.

The endpoint will not be removed for repository special signing key, but
it will reference the global signing key.

---------

Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

services/repository/files/update.go

@@ -686,7 +686,7 @@ func VerifyBranchProtection(ctx context.Context, repo *repo_model.Repository, do
 			}
 		}
 		if protectedBranch.RequireSignedCommits {
-			_, _, _, err := asymkey_service.SignCRUDAction(ctx, repo.RepoPath(), doer, repo.RepoPath(), branchName)
+			_, _, _, err := asymkey_service.SignCRUDAction(ctx, doer, repo.RepoPath(), branchName)
 			if err != nil {
 				if !asymkey_service.IsErrWontSign(err) {
 					return err