How to make form non required?

Solution: 

Good thing that plugin is using woocommerce_form_field function. In that case we can use it’s filter. This should do it.

add_filter( 'woocommerce_form_field_args', 'magee_table_number_field_args', 10, 3 );
function magee_table_number_field_args( $args, $key, $value ){
    if ( $key == 'mgrt_table_num_dine' ) {
        $args['required'] = false;
    }
    return $args;
}