In `ajax-actions.php`, remove dead code:

* In `wp_ajax_add_tag()`, `$post_type` is set and never used.
* In `wp_ajax_hidden_columns()`, `$hidden` is set twice, but only checks for the existence of `$_POST['hidden']` the first time. The two lines can be combined and work together.
* In `wp_ajax_inline_save()`, `$mode` is set and never used.
* In `wp_ajax_find_posts()`, `$searchand = $search = '';` is leftover cruft, neither variable is used. `$wpdb` does not need to be imported, it is never used.
* In `wp_ajax_wp_fullscreen_save_post()`, `$post_type` is set and never used.
* In `wp_ajax_save_attachment_order()`, `$post` is set and never used.
* In `wp_ajax_send_attachment_to_editor()`, `$title` is set and never used. 

See #27882.

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git-svn-id: http://core.svn.wordpress.org/trunk@28120 1a063a9b-81f0-0310-95a4-ce76da25c4cd