Remove the Prometheus HAProxy exporter image

- The source code is no longer maintained [1]
- HAProxy provides its own Prometheus metrics source [2]
- In Kolla Ansible, we have merged support for using the built in
  metrics source [3]

For these reasons, particularly the first, we should remove it immediately.

[1] https://github.com/prometheus/haproxy_exporter
[2] https://www.haproxy.com/blog/haproxy-exposes-a-prometheus-metrics-endpoint
[3] https://review.opendev.org/c/openstack/kolla-ansible/+/877118

Change-Id: Ia30e0fc367841df3726562b57b57df0f6fbb3a81

Author: dougszumski

docker/prometheus/prometheus-haproxy-exporter/Dockerfile.j2

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    HAProxy Prometheus metrics are now available directly from HAProxy. As
+    such, the standalone Prometheus HAProxy exporter was archived in March
+    2023, is no longer supported and the Docker image has been removed from
+    Kolla. For further details see `here
+    <https://www.haproxy.com/blog/haproxy-exposes-a-prometheus-metrics-endpoint>`__.