Fix for non-allocation of payment to fully reversed checkboxes option

Fix for non-allocation of payment to fully reversed checkboxes option

This addresses an issue partially identified in https://lab.civicrm.org/dev/financial/issues/98 whereby
civicrm_entity_financial_trxn records for the civicrm_financial_item table are not created when
refunding against line items with a quantity of zero  - ie where there were selected checkboxes & they
were reversed.

The reason this check was there relates to former logic - now we have a calculated allocation
we don't need this check.

I want to do a test but need to do a bit of work to get it set up & I think this should hit 5.20
as it relates to an issue identified by Kevin in reviewing patches merged to that version