`FederationClient.backfill`: stop flagging events as outliers (#11632)

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/231992

Fixes #8894.