debian: Remove kolla-ansible-debian-zun job

* When the Debian job got upgraded from Bullseye
  to Bookworm, the non-voting Zun job started failing.
* This is because Debian Bookworm doesn't support the
  required Docker 20 pinning.
* Until Zun gains support for newer dependencies this
  job will keep failing.

Change-Id: I1e1d9ea7c76d1b2f77b09b7948f138b88ef483d2