[stable-only] remove periodic-stable-jobs template

This project is marked as inactive, besides, the stable periodic jobs
are failing. This patch removes the template to free up CI resource.

The CI health can be checked anytime by proposing a patch against the
stable branch.

Conflicts:
  .zuul.yaml

Change-Id: I3d43519c34b8775a73a311216a945cf69cd89fe4
(cherry picked from commit f3254723a3)
(cherry picked from commit 602656efe0)

.zuul.yaml

@@ -4,7 +4,6 @@
       - check-requirements
       - openstack-cover-jobs
       - openstack-python3-zed-jobs
-      - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check: