ci: drop systemd-stable from advanced-commit-linter config

It's sufficient enough to check only the `systemd/systemd` repo.

Related to redhat-plumbers-in-action/advanced-commit-linter#62

rhel-only

Related: #2179309

.github/advanced-commit-linter.yml

@@ -2,7 +2,6 @@ policy:
   cherry-pick:
     upstream:
       - github: systemd/systemd
-      - github: systemd/systemd-stable
     exception:
       note:
         - rhel-only