Changed stats.js to use delete

Changed stats.js to use delete instead of setting the values to
undefined in order to save some memory.

stats.js

@@ -57,7 +57,7 @@ function flushMetrics() {
     conf.deleteCounters = conf.deleteCounters || false;
     for (key in metrics.counters) {
       if (conf.deleteCounters) {
-        metrics.counters[key] = undefined;
+        delete(metrics.counters[key]);
       } else {
         metrics.counters[key] = 0;
       }