add_filter( 'woocommerce_package_rates', 'snippetpress_hide_shipping_when_free_is_available', 100 );
function snippetpress_hide_shipping_when_free_is_available( $rates ) {
    $free_shipping = array();
    foreach ( $rates as $rate_id => $rate ) {
        if ( 'free_shipping' === $rate->method_id ) {
            $free_shipping[ $rate_id ] = $rate;
            break;
        }
    }
    return ! empty( $free_shipping ) ? $free_shipping : $rates;
}