zuul: drop devstack-gate reference

Devstack jobs no longer depend on the devstack-gate project, which has
been retired

Change-Id: Id4721d419b22b6d6498d192e3f313629ad33ef69
(cherry picked from commit 3e2a0ffe4f)
(cherry picked from commit 70f7bba359)
(cherry picked from commit db4b92fce4)
(cherry picked from commit c1b19c850a)

.gitreview

@@ -2,4 +2,4 @@
 host=review.opendev.org
 port=29418
 project=openstack/devstack-plugin-container.git
-defaultbranch=stable/xena
+defaultbranch=unmaintained/xena

.zuul.yaml

@@ -7,7 +7,6 @@
     timeout: 4200
     required-projects:
       - openstack/devstack
-      - openstack/devstack-gate
       - openstack/devstack-plugin-container
     vars:
       devstack_localrc:
@@ -25,7 +24,6 @@
     timeout: 7200
     required-projects:
       - openstack/devstack
-      - openstack/devstack-gate
       - openstack/devstack-plugin-container
     vars:
       devstack_services: