The refactoring of the AuthBackend code to use a registry mistakenly

used Registry.unregister_item(), which wasn't the method name that

was decided on (Registry.unregister()). This has been fixed and unit

tests have been added to ensure registration and unregistration through

the register_auth_backend and unregister_auth_backend methods works

correctly.
In addition, the AuthBackendRegistry.get method was removed becuase

the AuthBackendRegistry already inherits from

ExceptionFreeGetterMixin, which provides the same behaviour.

Ran unit tests.