Drop backwards compat hack for event serialization (#16069)

changelog.d/16069.misc

@@ -0,0 +1 @@
+Drop backwards compat hack for event serialization.

synapse/events/snapshot.py

@@ -186,9 +186,6 @@ async def serialize(self, event: EventBase, store: "DataStore") -> JsonDict:
             ),
             "app_service_id": self.app_service.id if self.app_service else None,
             "partial_state": self.partial_state,
-            # add dummy delta_ids and prev_group for backwards compatibility
-            "delta_ids": None,
-            "prev_group": None,
         }
 
     @staticmethod
@@ -203,21 +200,14 @@ def deserialize(storage: "StorageControllers", input: JsonDict) -> "EventContext
         Returns:
             The event context.
         """
-        # workaround for backwards/forwards compatibility: if the input doesn't have a value
-        # for "state_group_deltas" just assign an empty dict
-        state_group_deltas = input.get("state_group_deltas", None)
-        if state_group_deltas:
-            state_group_deltas = _decode_state_group_delta(state_group_deltas)
-        else:
-            state_group_deltas = {}
 
         context = EventContext(
             # We use the state_group and prev_state_id stuff to pull the
             # current_state_ids out of the DB and construct prev_state_ids.
             storage=storage,
             state_group=input["state_group"],
             state_group_before_event=input["state_group_before_event"],
-            state_group_deltas=state_group_deltas,
+            state_group_deltas=_decode_state_group_delta(input["state_group_deltas"]),
             state_delta_due_to_event=_decode_state_dict(
                 input["state_delta_due_to_event"]
             ),