Fix promotions

Problem/Motivation

Promotions are not handled correctly. This is follow up to #3168933: Incorrect price with product promotions in Paytrail where we removed promotion support.

Proposed resolution

1. Product collection should be disabled if order has any order level promotions.
2. Use OrderItem->getAdjustedUnitPrice() to calculate discounted unit prices (this should cover most promotions).

@todo