Don't include redundant prev_state in new events

Author: dakariakin

synapse/events/builder.py

@@ -167,7 +167,6 @@ async def build(
             "content": self.content,
             "unsigned": self.unsigned,
             "depth": depth,
-            "prev_state": [],
         }
 
         if self.is_state():

synapse/federation/federation_client.py

@@ -906,9 +906,6 @@ async def send_request(destination: str) -> Tuple[str, EventBase, RoomVersion]:
             # The protoevent received over the JSON wire may not have all
             # the required fields. Lets just gloss over that because
             # there's some we never care about
-            if "prev_state" not in pdu_dict:
-                pdu_dict["prev_state"] = []
-
             ev = builder.create_local_event_from_event_dict(
                 self._clock,
                 self.hostname,