if( ! function_exists( 'videotube_remove_comment_form_default_fields' ) ){
    /**
     * @param unknown_type $fields
     * @return unknown
     */
    function videotube_remove_comment_form_default_fields( $fields ) {
        unset( $fields['author'] );
        unset( $fields['email'] );
        unset( $fields['url'] );
        return $fields;
    }
    add_filter( 'comment_form_default_fields' , 'videotube_remove_comment_form_default_fields', 10, 1 );
}