Unset flat rate shipping if free is available

add_filter( 'woocommerce_package_rates', 'hide_frate_shipping_when_free_is_available', 10, 2 );

function hide_frate_shipping_when_free_is_available( $rates, $package ) {
    if ( isset( $rates['free_shipping'] ) ) {
        unset( $rates['flat_rate'] );
    }
    return $rates;
}