Remove a Gitaly allow_disk_access call

The implementation was fixed in this MR:
https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/21238

However, the allowlist and issue reference was never removed.

I also take the opportunity to add a comment explaining that the user
must have permission to update the branch, since that wasn't very clear
to me when reading through this code.

ee/app/services/projects/update_mirror_service.rb

@@ -142,10 +142,10 @@ module Projects
         newrev = upstream.dereferenced_target.sha
         oldrev = local.dereferenced_target.sha
 
-        # Gitaly migration: https://gitlab.com/gitlab-org/gitaly/issues/1246
-        ::Gitlab::GitalyClient::StorageSettings.allow_disk_access do
+        # If the user doesn't have permission to update the diverged branch
+        # (e.g. it's protected and the user can't force-push to protected
+        # branches), this will fail.
         repository.update_branch(branch_name, user: current_user, newrev: newrev, oldrev: oldrev)
-        end
       elsif branch_name == project.default_branch
         # Cannot be updated
         errors << "The default branch (#{project.default_branch}) has diverged from its upstream counterpart and could not be updated automatically."