Retire devstack-plugin-pika project

This plugin was used by the oslo.messaging project to test the
development of an alternative RabbitMQ message driver based on
the Pika library (instead of the default Kombu driver).

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

Depends-On: https://review.opendev.org/748712
Change-Id: I7ab16b6e0800bb509877066cf118419ae7e55d21

reference/legacy.yaml

@@ -709,6 +709,10 @@ oslo:
       retired-on: 2017-09-30
       repos:
         - openstack/pylockfile
+    devstack-plugin-pika:
+      retired-on: 2020-08-28
+      repos:
+        - openstack/devstack-plugin-pika
 
 Packaging-deb:
   ptl:

reference/projects.yaml

@@ -2112,10 +2112,6 @@ oslo:
     devstack-plugin-kafka:
       repos:
         - openstack/devstack-plugin-kafka
-    devstack-plugin-pika:
-      release-management: deprecated
-      repos:
-        - openstack/devstack-plugin-pika
     etcd3gw:
       repos:
         - openstack/etcd3gw