Bulk Removal of Post Thumbnails or Featured Images in a second
All you need to do is copy and paste below code in your theme’s functions.php file.
global $wpdb;
$wpdb->query( "
DELETE FROM $wpdb->postmeta
WHERE meta_key = '_thumbnail_id'
" );
Important Note: Please remove this code immediately after saving your functions.php file. You will not be able to set featured images in WordPress as this code will keep removing featured images from posts.