simple_current_order only gets incomplete orders

This currently causes an issue with better_spree_paypal_express, and
possibly other extensions.

While Spree sets the session's order_id to nil when an order is
complete, better_spree_paypal_express does not.  Therefore, a completed
order can be retrieved as the simple_current_order.

It makes sense to replicate the logic of current_order (which does not
allow completed orders to be returned) here.

Fixes #4702