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.

git-svn-id: https://develop.svn.wordpress.org/trunk@26483 602fd350-edb4-49c9-b593-d223f7449a82