Remove unused variable WAKE_UP

Change-Id: I65f57973b66f353a1e6fef3d0bf7793f821c1c5c
Closes-Bug: 1850259

oslo_messaging/_drivers/amqpdriver.py

@@ -44,6 +44,7 @@ class MessageOperationsHandler(object):
     serialized and run in the same thread, since underlying drivers like kombu
     are not thread safe.
     """
+
     def __init__(self, name):
         self.name = "%s (%s)" % (name, hex(id(self)))
         self._tasks = moves.queue.Queue()
@@ -386,8 +387,6 @@ class NotificationAMQPListener(AMQPListener):
 
 class ReplyWaiters(object):
 
-    WAKE_UP = object()
-
     def __init__(self):
         self._queues = {}
         self._wrn_threshold = 10