In WordPress, you can only remove post featured image one by one by editing each post and clicking on the “Remove Featured Image” link, which  is easier to do if you have only few posts but if you have hundreds of posts then removing them one by one can be a daunting task.  But fear not, there is a simpler and easier way to bulk remove  featured image in WordPress.

Copy the following code and paste it in your theme’s functions.php file.

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

Note: This code will only remove the featured image from the posts, It will not delete the images from your Media Library or from your uploads folder.

Copy the code and open your site’s Dashboard  and it will immediately remove all the featured image.  After that, please go back to your theme’s functions.php and remove the code from the file or else it will keep removing the featured images afterwards.

source: https://wpmunk.com/how-to-bulk-remove-featured-images-in-wordpress/