According to our "Supported build platforms" policy, we now do not support
Ubuntu 18.04 anymore. Remove the related container files and entries from
our CI.

Message-Id: <20220516115912.120951-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>