[Deprecate Zuora SOAP] Remove SOAP code related to listing subscription
Once the Zuora REST update is complete and the SOAP code is no longer needed, we will have plenty of unused legacy code that should be removed.
We can remove the Zuora SOAP API code and related feature flags. This issue focuses on cleaning up the SOAP logic and feature flags related to listing subscriptions which was added in #761 (closed).
A cleaner code base with less feature flags.