Retire the devstack-plugin-zmq project

This plugin was used by the oslo.messaging project to test the
development of a message driver based on the ZeroMQ library.

The jobs using this plugin were non-voting and non-gating.

Development of this driver was dropped in Rocky and is not
supported.

Depends-On: https://review.opendev.org/748724
Change-Id: I7c7ee35175c66e3f58860aa959775a0a8f78b481

reference/legacy.yaml

@@ -701,6 +701,10 @@ oslo:
         - openstack/oslosphinx
       tags:
         - stable:follows-policy
+    devstack-plugin-zmq:
+      retired-on: 2020-08-28
+      repos:
+        - openstack/devstack-plugin-zmq
     pylockfile:
       retired-on: 2017-09-30
       repos:

reference/projects.yaml

@@ -2095,10 +2095,6 @@ oslo:
       release-management: deprecated
       repos:
         - openstack/devstack-plugin-pika
-    devstack-plugin-zmq:
-      release-management: deprecated
-      repos:
-        - openstack/devstack-plugin-zmq
     etcd3gw:
       repos:
         - openstack/etcd3gw