No longer allow redundant calls to server start()

This has been deprecated since before Ocata.

Change-Id: I659cc690c0a5d4f82349c7c4882c34c9c82e8a8a
Closes-Bug: #1712397

oslo_messaging/server.py

@@ -395,13 +395,11 @@ class MessageHandlingServer(service.ServiceBase, _OrderedTaskRunner):
         choose to dispatch messages in a new thread, coroutine or simply the
         current thread.
         """
-        # Warn that restarting will be deprecated
         if self._started:
-            LOG.warning(_LW('Restarting a MessageHandlingServer is inherently '
+            LOG.warning(_LW('The server has already been started. Ignoring'
-                            'racy. It is deprecated, and will become a noop '
+                            ' the redundant call to start().'))
-                            'in a future release of oslo.messaging. If you '
+            return
-                            'need to restart MessageHandlingServer you should '
+
-                            'instantiate a new object.'))
         self._started = True
 
         executor_opts = {}