Stop publishing Xenial wheel mirror/cache content

We're trying to drop Xenial nodes entirely. Clean up the wheel
mirror/cache infrastructure in Zuul for Xenial as part of this.

Change-Id: Ide7f210bdbd69065fa123bebe4a6992a206c2a74

zuul.d/jobs.yaml

@@ -1104,27 +1104,6 @@
     vars:
       wheel_volume: 'mirror.wheel.bionica64'
 
-- job:
-    name: publish-wheel-cache-ubuntu-xenial
-    parent: build-wheel-cache-ubuntu-xenial
-    description: |
-      Publish Ubuntu Xenial wheels for OpenStack CI mirrors.
-    pre-run: playbooks/openafs-client/setup.yaml
-    post-run: playbooks/publish/wheel-mirror.yaml
-    allowed-projects:
-      - openstack/requirements
-    final: true
-    secrets:
-      - name: afs
-        secret: wheel_keytab
-
-- job:
-    name: release-wheel-cache-ubuntu-xenial
-    parent: release-wheel-cache
-    final: true
-    vars:
-      wheel_volume: 'mirror.wheel.xenialx64'
-
 - job:
     name: check-release-approval
     description: |

zuul.d/projects.yaml

@@ -4242,10 +4242,6 @@
         - release-wheel-cache-ubuntu-bionic:
             dependencies:
               - name: publish-wheel-cache-ubuntu-bionic
-        - publish-wheel-cache-ubuntu-xenial
-        - release-wheel-cache-ubuntu-xenial:
-            dependencies:
-              - name: publish-wheel-cache-ubuntu-xenial
         - publish-wheel-cache-centos-8-stream-arm64
         - release-wheel-cache-centos-8-stream-arm64:
             dependencies: