Remove unnecessary Appoint MVP feature flags

[oddball-lindsay]

Background

As a best practice, it has been advised that we remove any unnecessary feature flags after our Appoint MVP release.

Feature flags to consider:

• appoint_a_representative_enable_frontend (frontend)
• use_veteran_models_for_appoint (backend)
• appoint_a_representative_enable_pdf (backend)

Tasks

• Remove feature flags

Acceptance Criteria

• We are not keeping any unnecessary feature flags around, after our Appoint MVP release.