Add this code inside functions.php of the theme file. browse the site once. and delete the code again.
global $wpdb; $wpdb->query( " DELETE FROM $wpdb->postmeta WHERE meta_key = '_thumbnail_id' " );
Add this code inside functions.php of the theme file. browse the site once. and delete the code again.
global $wpdb; $wpdb->query( " DELETE FROM $wpdb->postmeta WHERE meta_key = '_thumbnail_id' " );