Issue #21 - Remove use counters telemetry

This reverts Bug 968923 - Implement some equivalent of Chrome's use counters (on top of telemetry?)

For reference: https://bugzilla.mozilla.org/show_bug.cgi?id=968923

dom/html/HTMLImageElement.h

@@ -261,12 +261,6 @@ public:
                                 const nsAString& aMediaAttr,
                                 nsAString& aResult);
 
-  /**
-   * If this image's src pointers to an SVG document, flush the SVG document's
-   * use counters to telemetry.  Only used for testing purposes.
-   */
-  void FlushUseCounters();
-
 protected:
   virtual ~HTMLImageElement();