Upgrader: Deprecate a few mostly unused functions, wp_update_plugin(), wp_update_theme(), and, wp_update_core().
wp_update_core() was still used, as it was never updated to make use of the newer Skins. Fixes #21874


git-svn-id: https://develop.svn.wordpress.org/trunk@25307 602fd350-edb4-49c9-b593-d223f7449a82