Remove unnecessary check from notifications.py

This check is unnecessary. If the class doesn't have callbacks then
it shouldn't have the resource type defined.

Change-Id: I1f4bc07354707ce3081f54e0a1f9cdfb1d655c08

keystone/notifications.py

@@ -337,8 +337,6 @@ def listener(cls):
             for resource_type, callbacks in resource_types.items():
                 # Make sure we register the provider for each event it
                 # cares to call back.
-                if not callbacks:
-                    continue
                 if not hasattr(callbacks, '__iter__'):
                     # ensure the callback information is a list
                     # allowing multiple callbacks to exist