In case you want to remove all the existing post thumbnails / feature images of a website, you can go to phpmyadmin and find your database, look up for wp_yoursite_postmeta table, and run this query:
DELETE FROM wp_yoursite_postmeta WHERE meta_key = '_thumbnail_id';
Remember, “yoursite” is your website’s prefix that you setup on your wp_config file.
This way is better than removing unwanted thumbnails one by one manually right?