[REF] simplify interaction with objects in complete order

[REF] simplify interaction with objects in complete order

Per https://github.com/civicrm/civicrm-core/pull/18382 this line seems to be heavily tested so
& per the test we can use the 'already calculated value' for payment_processor_id rather than look
it up in yet another place.

This adds a query but in most cases queries can be removed before calling this function as
the various objects are not really accessed anymore (apart from the few lines just above this)