fuzz/lnwire: minor touch-ups, remove MaxPayloadLength

This commit makes the fuzz/lnwire tests build and run without
crashing.

fuzz/lnwire/update_fail_htlc.go

@@ -11,10 +11,6 @@ func Fuzz_update_fail_htlc(data []byte) int {
 	// Prefix with MsgUpdateFailHTLC.
 	data = prefixWithMsgType(data, lnwire.MsgUpdateFailHTLC)
 
-	// Create an empty message so that the FuzzHarness func can check
-	// if the max payload constraint is violated.
-	emptyMsg := lnwire.UpdateFailHTLC{}
-
 	// Pass the message into our general fuzz harness for wire messages!
-	return harness(data, &emptyMsg)
+	return harness(data)
 }