Xena: drop prometheus-server image

It was dropped in xena, replaced by prometheus-v2-server.

Author: markgoddard

etc/kayobe/pulp.yml

@@ -275,7 +275,6 @@ stackhpc_pulp_images:
   - prometheus-mysqld-exporter
   - prometheus-node-exporter
   - prometheus-openstack-exporter
-  - prometheus-server
   - prometheus-v2-server
   - rabbitmq
   - redis