Drop Xenial wheel build job

This job ensures we could build wheels for the Xenial wheel
mirror/cache. Drop the jobs in preparation for removing Xenial entirely.

Depends-On: https://review.opendev.org/c/openstack/project-config/+/916954
Change-Id: I20df5db741e178f571c295715e1d418f32087536

zuul.d/jobs.yaml

@@ -1680,23 +1680,6 @@
       wheel-cache-ubuntu-bionic-arm64-python3:
         wheel_python: python3
 
-- job:
-    name: build-wheel-cache-ubuntu-xenial
-    parent: build-wheel-cache-base
-    description: |
-      Build Ubuntu Xenial wheels for OpenStack CI mirrors.
-    nodeset:
-      nodes:
-        - name: wheel-cache-ubuntu-xenial-python2
-          label: ubuntu-xenial
-        - name: wheel-cache-ubuntu-xenial-python3
-          label: ubuntu-xenial
-    host-vars:
-      wheel-cache-ubuntu-xenial-python2:
-        wheel_python: python2
-      wheel-cache-ubuntu-xenial-python3:
-        wheel_python: python3
-
 - job:
     name: openstack-multinode-fips
     abstract: true

zuul.d/project.yaml

@@ -28,10 +28,6 @@
             files:
               - playbooks/wheel-cache
               - roles/build-wheel-cache
-        - build-wheel-cache-ubuntu-xenial:
-            files:
-              - playbooks/wheel-cache
-              - roles/build-wheel-cache
         - build-wheel-cache-ubuntu-bionic:
             files:
               - playbooks/wheel-cache