Disable barbican plugin in octavia jobs

It is causing some failure in db upgrade and it's not
needed in our jobs as we have TLS disabled.

Change-Id: Ida43bd9b605dd048b9f4b010bf5b88fc9a8f0556

zuul.d/octavia.yaml

@@ -5,7 +5,6 @@
     parent: devstack-tobiko
     abstract: true
     required-projects:
-      - openstack/barbican
       - openstack/diskimage-builder
       - openstack/octavia
       - openstack/octavia-lib
@@ -13,7 +12,6 @@
       - openstack/tripleo-image-elements
     vars:
       devstack_plugins:
-        barbican: https://opendev.org/openstack/barbican.git
         octavia: https://opendev.org/openstack/octavia.git
       devstack_localrc:
         Q_AGENT: openvswitch