add_filter( 'woocommerce_package_rates', 'unset_shipping_when_free_is_available_all_zones', 9999, 2 );

function unset_shipping_when_free_is_available_all_zones( $rates, $package ) {

    $all_free_rates = array();

    foreach ( $rates as $rate_id => $rate ) {
        if ( 'free_shipping' === $rate->method_id ) {
            $all_free_rates[ $rate_id ] = $rate;
            break;
        }
    }

    if ( empty( $all_free_rates )) {
        return $rates;
    } else {
        return $all_free_rates;
    } 
}