Cleans up deprecated CSS sprite images (removed in WP 3.5)

References to old WP sprites are blowing up logs with 404 errors. PNGs have been removed from core since v3.5 or so. (See [ticket #20980](http://core.trac.wordpress.org/ticket/20980)).

New CSS mostly derived from [wp-includes/css/buttons.css](http://core.trac.wordpress.org/browser/branches/3.6/wp-includes/css/buttons.css).