Allow unshippable items on the order

Not every line item must be shipped on the order. I believe Spree should
be ok with that. Removing line items between delivery and payment steps
seems unexpected behaviour in that scenario

This was reported on #2966 and today I looked further and found that the reason for the remove line items before payment code was related to not allowing unshippable items on the final order.

@cmar could you please take a look?