Remove zuora_account_id from Customer model

Problem

In #4807 (closed), many Customer model (to be renamed User) references of zuora_account_id were feature flagged in favor of new logic using the new BillingAccount data structure. Once the feature flag has been vetted and promoted for production, the legacy code should be removed so that the zuora_account_id column on the customers table can be removed.

Proposal

Once the feature has been vetted, let's remove the legacy code that references zuora_account_id on Customer (to be renamed User). When we verify the column on the customers table is no longer used, it should be removed.