mxqsub: Remove default time warning

Remove the warning for the default runtime option in mxqsub. Users found
the message "option '--runtime' or '-t' not used. Your job will get
killed if it runs longer than the default of 15 minutes" annoying,
especially as there's no equivalent warning for the default memory
limit.

mxqsub.c

@@ -784,7 +784,7 @@ int main(int argc, char *argv[])
     arg_program_name   = NULL;
     arg_threads        = 1;
     arg_memory         = 2048;
-    arg_time           = 0;
+    arg_time           = 15;
     arg_max_per_node   = 0;
     arg_workdir        = current_workdir;
     arg_stdout         = "/dev/null";
@@ -1041,11 +1041,6 @@ int main(int argc, char *argv[])
         arg_program_name = argv[0];
     }
 
-    if (!arg_time) {
-        arg_time = 15;
-        mx_log_warning("option '--runtime' or '-t' not used. Your job will get killed if it runs longer than the default of %d minutes.", arg_time);
-    }
-
     if (arg_time > 60*24) {
         mx_log_warning("option '--runtime' specifies a runtime longer than 24h. Your job may get killed. Be sure to implement some check pointing.");
     }