Twenty Fourteen: delete the `featured_content_ids` transient on theme switch to make sure child themes can override the Featured Content quantity value. Also remove quantity parameter so child themes can change the amount of featured posts with `$max_posts`.

Props obenland, see #26660.
Built from https://develop.svn.wordpress.org/trunk@27120


git-svn-id: http://core.svn.wordpress.org/trunk@26987 1a063a9b-81f0-0310-95a4-ce76da25c4cd