Events returned by `backfill` should not be flagged as outliers.
Fixes:
```
AssertionError: null
File "synapse/handlers/federation.py", line 313, in try_backfill
dom, room_id, limit=100, extremities=extremities
File "synapse/handlers/federation_event.py", line 517, in backfill
await self._process_pulled_events(dest, events, backfilled=True)
File "synapse/handlers/federation_event.py", line 642, in _process_pulled_events
await self._process_pulled_event(origin, ev, backfilled=backfilled)
File "synapse/handlers/federation_event.py", line 669, in _process_pulled_event
assert not event.internal_metadata.is_outlier()
```
See https://sentry.matrix.org/sentry/synapse-matrixorg/issues/231992Fixes#8894.