Remove stream ordering from Metadata dict (#8452)

There's no need for it to be in the dict as well as the events table. Instead,
we store it in a separate attribute in the EventInternalMetadata object, and
populate that on load.

This means that we can rely on it being correctly populated for any event which
has been persited to the database.

synapse/events/utils.py

@@ -49,6 +49,11 @@ def prune_event(event: EventBase) -> EventBase:
         pruned_event_dict, event.room_version, event.internal_metadata.get_dict()
     )
 
+    # copy the internal fields
+    pruned_event.internal_metadata.stream_ordering = (
+        event.internal_metadata.stream_ordering
+    )
+
     # Mark the event as redacted
     pruned_event.internal_metadata.redacted = True