remove useless logging

There are many uselees logging in the decision log file:
Arguments dropped when creating context: {u'global_request_id': None}

Change-Id: I7583b2ff6ea1cc15173536ca1cf08cc9f0ecb20d
Closes-Bug: #1704736

watcher/common/context.py

@@ -63,6 +63,7 @@ class RequestContext(context.RequestContext):
         # oslo_context's RequestContext.to_dict() generates this field, we can
         # safely ignore this as we don't use it.
         kwargs.pop('user_identity', None)
+        kwargs.pop('global_request_id', None)
         if kwargs:
             LOG.warning('Arguments dropped when creating context: %s',
                         str(kwargs))