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.

fundingmanager.go

@@ -1712,7 +1712,7 @@ func (f *fundingManager) handleFundingSigned(fmsg *fundingSignedMsg) {
 		// Go on adding the channel to the channel graph, and crafting
 		// channel announcements.
 		lnChannel, err := lnwallet.NewLightningChannel(
-			nil, nil, completeChan, nil,
+			nil, completeChan, nil,
 		)
 		if err != nil {
 			fndgLog.Errorf("failed creating lnChannel: %v", err)
@@ -2005,7 +2005,7 @@ func (f *fundingManager) handleFundingConfirmation(peer lnpeer.Peer,
 
 	// We create the state-machine object which wraps the database state.
 	lnChannel, err := lnwallet.NewLightningChannel(
-		nil, nil, completeChan, nil,
+		nil, completeChan, nil,
 	)
 	if err != nil {
 		return err