remove enrollment policy UI for now

We will need to support three different enrollment policies:

• open: anyone can register
• closed: not accepting new accounts
• invite: requires an invite code

For now, we can remove this from the UI. In the future, rather then showing "enrollment policy" on the provider details page, we should just add a notice to that page if the enrollment policy is closed or invite:

For example:

• This provider is not accepting new users at this time.
• This provider requires an invite code to register a new account.

(from redmine: created on 2013-06-18, closed on 2013-06-19)