REST API: Remove a duplicate `require_once()`.

In `WP_REST_Attachments_Controller::create_item()`, `wp-admin/includes/image.php` was being `require_once()`-ed... twice. The superflous `require_once()` has been quietly removed.

Props david.binda.
Merges [44206] to trunk.
Fixes #45420.
Built from https://develop.svn.wordpress.org/trunk@44309


git-svn-id: https://core.svn.wordpress.org/trunk@44139 1a063a9b-81f0-0310-95a4-ce76da25c4cd