Hide shipping rates when free shipping is available in Woocommerce's Checkout

functions.php

<?php

add_filter('woocommerce_package_rates', 'maybe_force_free_shipping', 99);

/** 
 * @param array $rates Array of rates found for the package.
 * @return array
 */
function maybe_force_free_shipping($rates)
{
    $free = array();
    foreach ($rates as $rate_id => $rate) {
        if ($rate->method_id == 'free_shipping') {
            $free[$rate_id] = $rate;
            break;
        }
    }
    return ! empty($free) ? $free : $rates;
}