channel+cnct: remove preimage from channel and resolution

Now that the success resolver preimage field is always populated by the
incoming contest resolver, preimage lookups earlier in the
process (channel and channel arbitrator) can mostly be removed.

rpcserver.go

@@ -1917,7 +1917,7 @@ func (r *rpcServer) fetchActiveChannel(chanPoint wire.OutPoint) (
 	// we create a fully populated channel state machine which
 	// uses the db channel as backing storage.
 	return lnwallet.NewLightningChannel(
-		r.server.cc.wallet.Cfg.Signer, nil, dbChan, nil,
+		r.server.cc.wallet.Cfg.Signer, dbChan, nil,
 	)
 }