PERF: Release post_upload records when downloaded image is removed (#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.

app/models/post.rb

@@ -915,7 +915,6 @@ class Post < ActiveRecord::Base
       upload_ids << upload.id if upload.present?
     end
 
-    upload_ids |= Upload.where(id: downloaded_images.values).pluck(:id)
     post_uploads = upload_ids.map do |upload_id|
       { post_id: self.id, upload_id: upload_id }
     end