Remove redundant if-guard

The startswith("@") does the job

synapse/api/auth.py

@@ -521,7 +521,6 @@ class Auth(object):
 
         # Check state_key
         if hasattr(event, "state_key"):
-            if not event.state_key.startswith("_"):
             if event.state_key.startswith("@"):
                 if event.state_key != event.user_id:
                     raise AuthError(