Remove dead error check in WriteMessage.

The io.Writer.Write function always returns an error if the bytes written
is less than provided, so there is no reason to further check if the
payload length matches after a successful write.

message.go

@@ -196,15 +196,10 @@ func WriteMessage(w io.Writer, msg Message, pver uint32, btcnet BitcoinNet) erro
 	}
 
 	// Write payload.
-	n, err := w.Write(payload)
+	_, err = w.Write(payload)
 	if err != nil {
 		return err
 	}
-	if n != lenp {
-		str := fmt.Sprintf("failed to write payload - wrote %v bytes, "+
-			"payload size is %v bytes", n, lenp)
-		return messageError("WriteMessage", str)
-	}
 	return nil
 }