Remove code for unused no-SP registration heading

[aduth]

🎫 Ticket

Supports LG-9216

🛠 Summary of changes

Removes unreachable code for account registration headings for users not associated with a service provider.

SessionDecorator#registration_heading varies depending on whether there's an associated service provider, but this heading is only ever rendered in logic paths where a service provider exists. Thus, the changes here simplify to a single view for registrations associated with a service provider and removes unnecessary logic.

There should be no user-facing impact of these changes.

📜 Testing Plan

For app/views/devise/sessions/new.html.erb:

1. Start from sample application
2. Click "Sign in"
3. Observe heading "Example Sinatra App is using ..."

For app/views/users/authorization_confirmation/new.html.erb: See #5735 (comment)

[zachmargolis]

LGTM