[Deprecate Zuora SOAP] Remove SOAP code related to subscription updates
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 subscription updates which was added in #759 (closed). As suggested here, we can also remove the
Subscription.update class method as it is no longer needed with the REST implementation.
A cleaner code base with less feature flags.