rfqmsg: fix SCID calculation for sell accept

We only want a single place where SCIDs are calculated from the RFQ ID.
This is especially important due to a future change required where lnd
will only accept SCIDs in a certain range and we'll need to
deterministically shift the derived SCID to be within that range.

Author: guggero

rfqmsg/sell_accept.go

@@ -1,7 +1,6 @@
 package rfqmsg
 
 import (
-	"encoding/binary"
 	"fmt"
 
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -94,13 +93,7 @@ func newSellAcceptFromWireMsg(wireMsg WireMessage,
 // ShortChannelId returns the short channel ID associated with the asset sale
 // event.
 func (q *SellAccept) ShortChannelId() SerialisedScid {
-	// Given valid RFQ message id, we then define a RFQ short chain id
-	// (SCID) by taking the last 8 bytes of the RFQ message id and
-	// interpreting them as a 64-bit integer.
-	scidBytes := q.ID[24:]
-
-	scidInteger := binary.BigEndian.Uint64(scidBytes)
-	return SerialisedScid(scidInteger)
+	return q.ID.Scid()
 }
 
 // ToWire returns a wire message with a serialized data field.