Remove all featured images

Un-associate featured images

global $wpdb;

$attachments = $wpdb->get_results( "
     SELECT * 
     FROM $wpdb->postmeta 
     WHERE meta_key = '_thumbnail_id'
" );

foreach ( $attachments as $attachment ) {
    wp_delete_attachment( $attachment->meta_value, true );
}

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

(For WordPress)

Place in functions.php > Save > remove