CI: drop ubuntu-aarch64 job

Rabbitmq requires Erlang 22.3+ while Ubuntu/aarch64 has 22.2 one.

There is no interest in using Ubuntu/aarch64 images so there is no sense
in keeping always failing CI job.

Change-Id: Ia45f01be6724ee60d5993b2ae73648a278fbf6e6

.zuul.d/ubuntu.yaml

@@ -13,9 +13,6 @@
         - kolla-ansible-ubuntu-source-upgrade:
             vars:
               kolla_python_version: 3.8
-    check-arm64:
-      jobs:
-        - kolla-build-ubuntu-source-aarch64
     gate:
       queue: kolla
       jobs:
@@ -110,12 +107,6 @@
       base_distro: ubuntu
       install_type: source
 
-- job:
-    name: kolla-build-ubuntu-source-aarch64
-    parent: kolla-build-ubuntu-source
-    nodeset: kolla-ubuntu-focal-aarch64
-    voting: false
-
 - job:
     name: kolla-publish-ubuntu-source
     parent: kolla-build-ubuntu-source