Drop some wrapper functions

* `get_attached_audio()` (use `get_attached_media( 'audio' )`)
* `get_attached_images()` (use `get_attached_media( 'image' )`)
* `get_attached_videos()` (use `get_attached_media( 'video' )`)

props wonderboymusic. fixes #23843.

git-svn-id: https://core.svn.wordpress.org/trunk@24571 1a063a9b-81f0-0310-95a4-ce76da25c4cd