Themes: Prepare wp-admin.css for a merge with theme.css.

* Remove styles for `#current-theme` and `.theme-options`. Markup removed in [26141].
* Move "Tabbed Admin Screen Interface" section out of "Themes" section. See [26466].

see #25966.
Built from https://develop.svn.wordpress.org/trunk@26483


git-svn-id: http://core.svn.wordpress.org/trunk@26380 1a063a9b-81f0-0310-95a4-ce76da25c4cd