In `WP_Media_List_Table`, remove dead code:

* In `->prepare_items()`, importing `$wpdb` is unnecessary.
* In `->get_columns()`, `$taxonomies` is set twice before it is used.
* In `->display_rows()`, `$taxonomy_object` is set and never used.

See #27882.


Built from https://develop.svn.wordpress.org/trunk@28293


git-svn-id: http://core.svn.wordpress.org/trunk@28121 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Custom Header: Fix logic when a theme doesn't set `default-text-color`.

fixes #28042.
Built from https://develop.svn.wordpress.org/trunk@28294


git-svn-id: http://core.svn.wordpress.org/trunk@28122 1a063a9b-81f0-0310-95a4-ce76da25c4cd