Simplify parameters passed to completeOrder

Simplify parameters passed to completeOrder

Now that completeOrder has had most interaction with objects var cleaned up
we know only 2 objects are actually used
- contribution - which we can fetch in-function
- paymentProcessor - which we were actually only guessing

This alters it to only load & pass contribution. The value of passing in
processor seems lost in time. This code can only be reached for pay_later contributions - see

CRM_Contribute_BAO_Contribution::checkOnlinePendingContribution
and specifically
https://github.com/civicrm/civicrm-core/blob/318c63045e82d30ed7fe8cdacb91a6d31fc5331b/CRM/Contribute/BAO/Contribution.php#L2033

and only when submitted via an online event (same function) so by definition there IS NO PAYMENT PROCESSOR that applies.

Given that we can just do the minimum to load the contribution and pass that in.