witness_beacon: do not look up invoice preimages

This commit isolates preimages of forwarded htlcs from invoice
preimages. The reason to do this is to prevent the incoming contest
resolver from settling exit hop htlcs for which the invoice isn't marked
as settled.

server.go

@@ -374,7 +374,6 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB, cc *chainControl,
 	}
 
 	s.witnessBeacon = &preimageBeacon{
-		invoices:    s.invoices,
 		wCache:      chanDB.NewWitnessCache(),
 		subscribers: make(map[uint64]*preimageSubscriber),
 	}