Stop populating events.content

This field is no longer read from, so we should stop populating it. Once we're
happy that this doesn't break everything, and a rollback is unlikely, we can
think about dropping the column.

synapse/storage/events.py

@@ -1189,7 +1189,6 @@ class EventsStore(EventsWorkerStore):
                     "type": event.type,
                     "processed": True,
                     "outlier": event.internal_metadata.is_outlier(),
-                    "content": encode_json(event.content).decode("UTF-8"),
                     "origin_server_ts": int(event.origin_server_ts),
                     "received_ts": self._clock.time_msec(),
                     "sender": event.sender,