[CI] Drop the TripleO job

Per [1] TripleO relies on Kolla no longer.
That explains why the TripleO job is RED this late in the cycle.
This patch removes it from Kolla's pipeline to save resources.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-September/017008.html

Change-Id: I4b3b73e7e1b19ce9c6458c0a8488e0af8ff51d80

.zuul.d/centos.yaml

@@ -7,14 +7,6 @@
         - kolla-ansible-centos8-source
         - kolla-ansible-centos8-binary
         - kolla-ansible-centos8-source-upgrade
-        - tripleo-build-containers-centos-8:
-            voting: false
-            files:
-              - ^docker/.*$
-              - ^kolla/.*$
-              - ^requirements.txt$
-              - ^setup.cfg$
-              - ^setup.py$
         - kolla-ansible-centos8-source-bifrost:
             files: ^docker\/(base|bifrost|openstack-base)\/.*
         # Test rabbitmq & mariadb changes in multinode ceph jobs.