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.

peer.go

@@ -429,8 +429,7 @@ func (p *peer) QuitSignal() <-chan struct{} {
 func (p *peer) loadActiveChannels(chans []*channeldb.OpenChannel) error {
 	for _, dbChan := range chans {
 		lnChan, err := lnwallet.NewLightningChannel(
-			p.server.cc.signer, p.server.witnessBeacon, dbChan,
-			p.server.sigPool,
+			p.server.cc.signer, dbChan, p.server.sigPool,
 		)
 		if err != nil {
 			return err
@@ -1789,8 +1788,7 @@ out:
 			// set of active channels, so we can look it up later
 			// easily according to its channel ID.
 			lnChan, err := lnwallet.NewLightningChannel(
-				p.server.cc.signer, p.server.witnessBeacon,
-				newChan, p.server.sigPool,
+				p.server.cc.signer, newChan, p.server.sigPool,
 			)
 			if err != nil {
 				p.activeChanMtx.Unlock()