add_filter('woocommerce_cart_subtotal', 'custom_calculate_subtotal', 10, 3);

function custom_calculate_subtotal( $subtotal, $compound, $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
        return $subtotal;
    }

    $fees_total = 0;
    foreach ( WC()->cart->get_fees() as $fee ) {
        $fees_total += $fee->amount;
    }

    return wc_price( WC()->cart->subtotal - $fees_total );
}