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

Periodic stable jobs are failing for a long time since the project is
not actively maintained. This patch removes the periodic-stable-jobs
template to free up CI resource.

Conflicts:
  .zuul.yaml

Change-Id: I56428c7fed44c8e0970e4ae5c4a46a3c96bdcca4
(cherry picked from commit 6b3211357b)
(cherry picked from commit 20874f920e)
(cherry picked from commit 972686aa3e)
(cherry picked from commit 0bd8f3dcbb)

.zuul.yaml

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