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/lnwire_test.go

@@ -250,9 +250,6 @@ func TestEmptyMessageUnknownType(t *testing.T) {
 // TestLightningWireProtocol uses the testing/quick package to create a series
 // of fuzz tests to attempt to break a primary scenario which is implemented as
 // property based testing scenario.
-//
-// Debug help: when the message payload can reach a size larger than the return
-// value of MaxPayloadLength, the test can panic without a helpful message.
 func TestLightningWireProtocol(t *testing.T) {
 	t.Parallel()
 
@@ -274,9 +271,9 @@ func TestLightningWireProtocol(t *testing.T) {
 		// the 2 bytes for the message type) is _below_ the specified
 		// max payload size for this message.
 		payloadLen := uint32(b.Len()) - 2
-		if payloadLen > msg.MaxPayloadLength(0) {
+		if payloadLen > MaxMsgBody {
 			t.Fatalf("msg payload constraint violated: %v > %v",
-				payloadLen, msg.MaxPayloadLength(0))
+				payloadLen, MaxMsgBody)
 			return false
 		}