fixup! mxqkill: Use cancel_group

With the new logic, the normal program flow runs until the end of
main(). Unfortunately, there is a `return(1);` at the end. Remove that.

mxqkill.c

@@ -461,5 +461,4 @@ int main(int argc, char *argv[])
 
     mx_mysql_finish(&mysql);
     mx_log_info("MySQL: Connection to database closed.");
-    return 1;
 }