Remove deprecated executor in message handling servers

Removes the deprecated message executor when creating both RPC
and notification server instances. This parameter is deprecated[1],
as well eventlet option.
When not defined, the server will get the one that fits better the
current context (monkey patched or not)[2]

[1] 27d833e374
[2] 412ab4de92/oslo_messaging/_utils.py (L87)

Change-Id: I784407aa7db10bddcec5dc663e1cec65174631e0

watcher/common/rpc.py

@@ -137,7 +137,6 @@ def get_server(target, endpoints, serializer=None):
         TRANSPORT,
         target,
         endpoints,
-        executor='eventlet',
         serializer=serializer,
         access_policy=access_policy
     )
@@ -151,7 +150,6 @@ def get_notification_listener(targets, endpoints, serializer=None, pool=None):
         targets,
         endpoints,
         allow_requeue=False,
-        executor='eventlet',
         pool=pool,
         serializer=serializer
     )