[CI] Remove periodic jobs from unmaintained branch

There is no need to run periodics in unmaintained branches, they just
consume resource and teams are not checking the results.

Conflicts:
  .zuul.d/project.yaml

Change-Id: Ib25929f770d7c53db4215ac34556690a60b90528
(cherry picked from commit cf39ff71ec)

.zuul.d/project.yaml

@@ -6,7 +6,6 @@
       - horizon-nodejs14-jobs
       - horizon-non-primary-django-jobs
       - openstack-python3-yoga-jobs
-      - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:
@@ -28,7 +27,3 @@
         - horizon-integration-tests-xstatic-master
         - horizon-tox-py36-xstatic-master
         - horizon-nodejs14-run-test-xstatic-master
-    periodic:
-      jobs:
-        - horizon-nodejs14-run-test
-        - horizon-integration-tests