use delete for removing events after removing the final listener

lib/events.js

@@ -91,8 +91,10 @@ process.EventEmitter.prototype.removeListener = function (type, listener) {
     var i = list.indexOf(listener);
     if (i < 0) return this;
     list.splice(i, 1);
+    if (list.length == 0)
+      delete this._events[type];
   } else if (this._events[type] === listener) {
-    this._events[type] = null;
+    delete this._events[type];
   }
 
   return this;