HAProxy exposes a Prometheus metrics endpoint, it just needs to be
enabled. Enable this and remove configuration for
prometheus-haproxy-exporter. Remaining prometheus-haproxy-exporter
containers will automatically be removed.

Change-Id: If6e75691d2a996b06a9b95cb0aae772db54389fb
Co-Authored-By: Matt Anson <matta@stackhpc.com>