Remove added userActivity_ to global

I browsed this html.
```html
<!DOCTYPE html>
<html>
  <head>
    <script src="video.js/dist/video-js/video.dev.js"></script>
  </head>
  <body>
    <div id="test"></div>
    <div id="video"></div>
    <script>
      videojs.options.techOrder = ['flash'];
      videojs.options.flash.swf = 'video.js/dist/video-js/video-js.swf';
      videojs('video');

      var el = document.getElementById('test');
      el.style.cssText = 'border: 1px solid red; background-color: #fee; font-weight: bold;';
      setInterval(function() {
        el.textContent = 'window.userActivity_ is "' + window.userActivity_ + '"';
      }, 500);
    </script>
  </body>
</html>
```

```window.userAcitvity_``` is undefined at first. But it is now true If I click on the video.
Because ```onMouseActivity();``` is not specified thisValue, thisValue in the ```reportUserActivity``` is window.