The set_and_forget setting is no longer needed

Problem/Motivation

Once we complete 📌 RegistrationIsOpen constraint should add cache expiry metadata Active and 📌 Use HostEntity::isAvailableForRegistration in RegisterAccessCheck Active we will have properly functioning access for register links and forms.
At this point there's no obvious need for the set_and_forget setting.

Probably we should remove it, to reduce complexity. This would have the desirable effect of clarify the semantic of 'status': its something manually set by the admin that overrides other settings, not just an implementation mechanism for the close setting.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes