Remove hook_menu_site_status_alter() in favor of request listeners

I honestly have no idea what this hook is even for, other than this implementation:

http://api.drupal.org/api/drupal/core!modules!user!user.module/function/user_menu_site_status_alter/8

But that should get ripped out and put into the MaintenanceModeSubscriber, which (I think) makes more sense than keeping it separate anyway.

The other implementations of this hook should turn into something more sensible, too.