code: protected synchronized void unsetCustomUserRegistry(Map<String, Object> props) {
          String id = getId(props);
          customUserRegistries.remove(id);
          ServiceRegistration<UserRegistry> registration = registrynRegistrationsToUnregister.remove(id);
          if (registration != null) {
              registration.unregister();
          }
      }
docs: Method will be called for each com.ibm.websphere.security.UserRegistry that is
  unregistered in the OSGi service registry. We must remove this instance
  from our internal set of listeners.
  
  @param ref Reference to an unregistered com.ibm.websphere.security.UserRegistry