Remove fallbacks for service users resolution

ServiceUserMapperImpl has several levels of fallback for service user resolution (fallback to bundle default, or to global default). While this offers a lot of flexibility, it introduces non-determinism in a security feature. If defaults are set, it can happen (especially at startup) that code is executed using different serviceUsers, e.g. a component can execute using the bundle default or global default until its specific subService is available, and it can be easily imagined how this can cause subtle errors.