fix a bug which causes activitystream actor assignment to be invalid

Resolves #4660

awx/main/middleware.py

@@ -68,7 +68,6 @@ class ActivityStreamMiddleware(threading.local):
                 if user.exists():
                     user = user[0]
                     instance.actor = user
-                    instance.save(update_fields=['actor'])
             else:
                 if instance.id not in self.instance_ids:
                     self.instance_ids.append(instance.id)