Unable to delete images and blocks if parent is global asset folder

Description

Somehow, you have blocks and/or media in the "For this page" folder that you cannot delete. To verify this, create a new page and look in the "For this page" folder. If it has a block or media file before you add anything, you're affected by this bug. This forum thread reports images that are always visible on newly-created pages and blocks:

http://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2017/4/when-creating-a-new-page-in-cms-i-get-default-media-items-in-quotfor-this-pagequot/

This bug fix lets users cut/copy/delete non-folder content that has the content asset root as parent link.