[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: Ib896b24e71e81107c5c104923b3bdec37e069613
(cherry picked from commit 1da3fcd72d)
(cherry picked from commit 02447d4291)

.zuul.yaml

@@ -3,7 +3,6 @@
       - check-requirements
       - openstack-python-jobs
       - openstack-python35-jobs
-      - periodic-stable-jobs
     check:
       jobs:
         - openstack-tox-lower-constraints