MaxScale logs warnings about missing listeners for sub-services

The following warnings is logged for the service service1 if it doesn't have a listener.

warning: Service 'service1' has no listeners defined.

While this might make some sense for simple single-layer services, the warnings is misleading for services that are being used as targets for other services.

The warning isn't that helpful in the first place which is why I believe removing it might be a sensible approach to this. If it is kept, the warning should only be logged if no other service uses the service in question. This way the warning would only be logged for top-level services in a multi-service configuration.