Drop focal platform job and support

This was dropped in tempest, too[0], and we want to focus on getting and
keeping the jammy job stable.

Still retaining the nodeset definitions until we are sure they are not
needed in other projects.

[0] https://review.opendev.org/c/openstack/tempest/+/884952

Change-Id: Iafb5a939a650b763935d8b7ce7069ac4c6d9a95b
2 files changed