[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.

Change-Id: Ib896b24e71e81107c5c104923b3bdec37e069613
(cherry picked from commit 1da3fcd72d)

.zuul.yaml

@@ -5,4 +5,3 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
-      - periodic-stable-jobs