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

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git-svn-id: http://core.svn.wordpress.org/trunk@25269 1a063a9b-81f0-0310-95a4-ce76da25c4cd