To remove/unset the featured images from all posts in a simple way just paste the following code in the functions.php file of your theme. It’s a database query and the change will take effect as soon as you save the file.

Please note:

• This change is irreversible.
• It will remove all featured images from all posts.
• It will not remove the uploaded images in the media library.
• You must remove the code from the theme’s functions.php file before closing the functions.php file. If you do not do this, when you select an image as a featured image again, it will not appear as it will continue to be deleted.

global $wpdb;
$wpdb->query( "
    DELETE FROM $wpdb->postmeta 
    WHERE meta_key = '_thumbnail_id'
" );