PERF: Release post_upload records when downloaded image is removed (PR #10379)

Previously we would unconditionally keep all images downloaded via pull_hotlinked_images, even if they are later removed from the post. This commit removes that logic, and relies on the existing link_post_uploads process to pick up the downloaded images in cooked. Specs are added to ensure this is working correctly for regular hotlinked images, and for oneboxes.


Works great!