This change remvoes the az filter and always enabled
the placement pre-filter. As part of this removal
the config option to control enabling the pre-filter
is removed as it is now mandatory.
The AZ filter docs and tests are also removed and an upgrade
release note is added.
Depends-On: https://review.opendev.org/c/openstack/devstack/+/886972
Change-Id: Icc8580835beb2b4d40341f81c25eb1f024e70ade