Remove xenial-arm64 wheel jobs

Nothing uses Xenial on ARM64, and we don't want anything to start, so
let's remove the nodes.  This removes the wheel publishing jobs.

Change-Id: Ia44c384072c0482cfd11c642013fd51004f85c8b

zuul.d/jobs.yaml

@@ -990,20 +990,6 @@
       - name: afs
         secret: wheel_keytab
 
-- job:
-    name: publish-wheel-cache-ubuntu-xenial-arm64
-    parent: build-wheel-cache-ubuntu-xenial-arm64
-    description: |
-      Publish Ubuntu Xenial ARM64 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
     description: |

zuul.d/projects.yaml

@@ -5191,7 +5191,6 @@
         - publish-wheel-cache-debian-buster-arm64
         - publish-wheel-cache-ubuntu-focal-arm64
         - publish-wheel-cache-ubuntu-bionic-arm64
-        - publish-wheel-cache-ubuntu-xenial-arm64
         - release-wheel-cache:
             dependencies:
               - publish-wheel-cache-centos-7
@@ -5204,7 +5203,6 @@
               - publish-wheel-cache-debian-buster-arm64
               - publish-wheel-cache-ubuntu-focal-arm64
               - publish-wheel-cache-ubuntu-bionic-arm64
-              - publish-wheel-cache-ubuntu-xenial-arm64
         - propose-updates:
             nodeset: ubuntu-bionic
             branches: master