Fix PHP Warning (for 5.4 runtime)

Line 88 `$_SESSION['shipping']['title'] = 'free_free';` raised this warning: 

[12-Apr-2013] PHP Warning:  Illegal string offset 'title' in /shop/includes/modules/pages/checkout_shipping/header_php.php on line 88

Since line 87 `$_SESSION['shipping'] = 'free_free';` it seems like removal is warranted.