Removed unnecessary tabindex = 0 from <a /> - they get it by default.

common/lib/xmodule/xmodule/js/src/video/07_video_volume_control.js

@@ -61,9 +61,6 @@ function () {
             slide: state.videoVolumeControl.onChange
         });
 
-        // Make sure that we can focus the actual volume slider while Tabing.
-        state.videoVolumeControl.volumeSliderEl.find('a').attr('tabindex', '0');
-
         state.videoVolumeControl.el.toggleClass('muted', state.videoVolumeControl.currentVolume === 0);
     }