Deleting a room does not delete unreferenced state groups in the room.

[erikjohnston]

Instead, it only deletes referenced state groups in the room.

We should modify:

synapse/synapse/storage/controllers/purge_events.py

Lines 45 to 46 in e80dad5

	state_groups_to_delete = await self.stores.main.purge_room(room_id)
	await self.stores.state.purge_room_state(room_id, state_groups_to_delete)

To also fetch state groups from state_groups that match the room ID being deleted.