Cleanup: drop unused `payment_instrument_id` field

Creditor doesn't have this field anymore in the New World Order.
Cleanup: drop unused variable
Don't rely on $form->_values in postProcess hook

This is only available when editing an existing Payment Processor
record, but not when creating a new one. Thus, we need to use other ways
to get at the values:

 * 'user_name' is available from $form->_submitValues

 * The Payment Processor Type can be fetched by the ID in $form->_ppType
   using the API. (We could also get the ID from _submitValues -- but as
   we get the convenient _ppType too, let's use it :-) )

 * The 'payment_processor_id' (i.e. the ID of the newly created PP
   record) is not available anywhere in the postProcess hook. Instead,
   we have to fetch the record from the DB (by the uniqe `name`) using
   the API.
Only try retrieving existing creditor when editing existing PP record

Don't run the API 'get' with an unset $ppid, as this gets all existing
creditor records, rather than getting none. (Resulting in bogus values
being pre-filled when adding a new record.)
Merge pull request #86 from sfe-ev/creditor_save_fix

Two buttons is one too many for my wee brain.