Cleans up deprecated CSS sprite images (removed in WP 3.5) -- For 2.4 branch

Resubmitting for 2.4 branch...

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).