Remove unnecessary todo

Remove unnecessary todo

The ChannelMonitor already monitors the chain for counterparties
revealing preimages, and will give the HTLCSources back to the
ChannelManager for claiming. Thus it's unnecessary for the ChannelManager
to monitor these HTLCs itself.

See is_resolving_htlc_output:
- if the counterparty broadcasted and then claimed one of the HTLCs we
  offered them, line 2015 is where the ChannelMonitor gives the ChannelManager
  the HTLC source
- if we broadcasted and they claimed an HTLC we offered them, line 2025 is
  where the ChannelMonitor gives the ChannelManager the HTLC source