Replace non-functional 'wp_audio_shortcode_handler' and 'wp_video_shortcode_handler' filters with 'wp_audio_shortcode_override' and 'wp_video_shortcode_override'.

props ericlewis, SergeyBiryukov.
fixes #25313.

git-svn-id: https://develop.svn.wordpress.org/trunk@25484 602fd350-edb4-49c9-b593-d223f7449a82