lnwire: remove MaxPayloadLength from Message interface

Removes the MaxPayloadLength function from the Message interface
and checks that each message payload is not greater than MaxMsgBody.
Since all messages are now allowed to be 65535 bytes in size, the
MaxPayloadLength is no longer needed.

lnwire/message.go

@@ -151,7 +151,6 @@ type Serializable interface {
 type Message interface {
 	Serializable
 	MsgType() MessageType
-	MaxPayloadLength(uint32) uint32
 }
 
 // makeEmptyMessage creates a new empty message of the proper concrete type
@@ -237,19 +236,12 @@ func WriteMessage(w io.Writer, msg Message, pver uint32) (int, error) {
 	payload := bw.Bytes()
 	lenp := len(payload)
 
-	// Enforce maximum overall message payload.
-	if lenp > MaxMessagePayload {
+	// Enforce maximum message payload, which means the body cannot be
+	// greater than MaxMsgBody.
+	if lenp > MaxMsgBody {
 		return totalBytes, fmt.Errorf("message payload is too large - "+
-			"encoded %d bytes, but maximum message payload is %d bytes",
-			lenp, MaxMessagePayload)
-	}
-
-	// Enforce maximum message payload on the message type.
-	mpl := msg.MaxPayloadLength(pver)
-	if uint32(lenp) > mpl {
-		return totalBytes, fmt.Errorf("message payload is too large - "+
-			"encoded %d bytes, but maximum message payload of "+
-			"type %v is %d bytes", lenp, msg.MsgType(), mpl)
+			"encoded %d bytes, but maximum message body is %d bytes",
+			lenp, MaxMsgBody)
 	}
 
 	// With the initial sanity checks complete, we'll now write out the