fullscreen-zindex: Revert to CSS-set z-index when exiting fullscreen

Instead of setting the z-index to 'auto', remove the style added via the
.css() call to reuse the value set in the CSS stylesheets.
Merge pull request #53 from edx-solutions/fullscreen-zindex

fullscreen-zindex: Revert to CSS-set z-index when exiting fullscreen