[FIX] point_of_sale: allow refunds on non-cash journals

A customer can pay an order with a non-cash journal (e.g. a credit
card through a stand-alone terminal).

When this order has to be refunded later it should be possible to do
so on that same journal.

Some code prevented this from happening, notably a check in
order_is_valid which explicitly prevented this for an unknown
reason. There is no explanation in the
commit (4647f896) introducing that.

This commit allows the following:

1. input what you want to refund with a negative qty
2. click 'Payment'
3. click the Credit Card (type Bank) journal
4. (amount is auto-populated)
5. click 'Validate'

opw-805302