volume, mixer: removed the "relative" parameter

Since the "volume" command has been removed, nobody uses relative
volumes anymore.

src/command.c

@@ -1061,7 +1061,7 @@ handle_setvol(struct client *client, G_GNUC_UNUSED int argc, char *argv[])
 	if (!check_int(client, &level, argv[1], need_integer))
 		return COMMAND_RETURN_ERROR;
 
-	success = volume_level_change(level, 0);
+	success = volume_level_change(level);
 	if (!success) {
 		command_error(client, ACK_ERROR_SYSTEM,
 			      "problems setting volume");