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.

Mark any events that get persisted via `_auth_and_persist_outliers` as, well,
outliers.

Currently this will be a no-op as everything will already be flagged as an
outlier, but I'm going to change that.

The events are now flagged as outliers later on, by `_auth_and_persist_outliers`.

The events created here are returned in the result of `send_join` to
`FederationHandler.do_invite_join`. From there they are passed into
`FederationEventHandler.process_remote_join`, which passes them to
`_auth_and_persist_outliers`... which sets the `outlier` flag.

stop flagging the events returned by `get_event_auth` as outliers. This method
is only called by `_get_remote_auth_chain_for_event`, which passes the results
into `_auth_and_persist_outliers`, which will flag them as outliers.

we pass all the events into `_auth_and_persist_outliers`, which will now flag
the events as outliers.

This param is now never set to True, so we can remove it.

This is no longer set anywhere, so we can remove it.

... and chase it down into `get_pdu_from_destination_raw`.

This is never set to `True`, so can be removed.

…st_outliers