removed "unloadable" declarations

They were causing circular dependency errors in development when
classes changed and had to be recompiled.  Turns out they're no longer
needed for Rails 4:

   https://github.com/thoughtbot/clearance/issues/276
   https://github.com/thoughtbot/clearance/pull/277

In development if I change one of the classes it recompiles, even
without the "unloadable" declaration, and I no longer get circular
dependency errors.