Remove stale todo in AuthenticationManager

The way forward on a Drupal core blocker is still be discussed...

#2456283: AuthenticationManager::getSortedProviders is protected
#2490228: Add Authentication Collector
#2228141: Add authentication support to REST views

In short the methods on the AuthenticationManager are still fluid.

I am just trying to separate out on un-controversial step, to remove a @todo, related to a closed issue, just to avoid confusion.