Stop tracking funding outpoints for in-flight monitor updates

We no longer need the funding outpoints as we can fully rely on the
channel IDs instead to track them. This acts as a breaking change, as we
will no longer write the legacy version of in-flight monitor updates,
preventing downgrades to versions that only supported such. Along the
way, we clean up `handle_new_monitor_update` and
`MonitorUpdateRegeneratedOnStartup` now that their `funding_txo`
parameter is unused.