Promote `account_data_structure` feature and remove feature flag checks

Background

As noted in the feature flag rollout issue, the account_data_structure feature flag has been enabled in all environments. It was enabled in Production on Monday, 2023-02-06. We haven't noticed any major issues so far that have required disabling the feature flag and we will continue to monitor it.

Proposal

If after a week (anytime after 2023-02-13), the feature flag is still stable, we should feel comfortable promoting the account_data_structure feature flag and removing the legacy code and conditions from the application code.

While removing the feature flag checks, we may encounter opportunities to refactor and simplifying existing logic.