Never downgrade cells from RELAY_EARLY to RELAY.
To support v1 connections, we had a rule that any RELAY_EARLY cell sent over them had to be downgraded and sent as RELAY. Since 0.2.0.x is supposed to be dead, we can remove this. It will be needed to make #4339 (moved) work.
See #4339 (moved) for related issues. Based on a comment by wanoskarnet.