Remove hardcoded link to cart which stops use of checkout without cart

We are currently implementing a custom checkout process that doesn't use the cart. So we initiate the checkout process by visiting checkout/%commerce_order, rather than the cart working it out.

However, the following code hardcodes a link with the cart submodule.

commerce_checkout_progress.module Line 153

  // Option to display back pages as links.
  if ($link) {
    // Load the *shopping cart* order. It gets deleted on last page.
    if ($order = commerce_cart_order_load($GLOBALS['user']->uid)) {
      $order_id = $order->order_id;
    }
  }

I think it should be reasonably easy to work it out, I'll create a possible patch.