Removes duplicate longhorn-kafka StorageClass managed by Kafka chart.
All applications now use single 'longhorn' StorageClass (3 replicas, Immediate binding).
Changes:
- Kafka chart: use 'longhorn' instead of 'longhorn-kafka'
- Delete Kafka StorageClass template (no longer needed)
- Update longhorn-storageclass.yaml to match deployed config (Immediate, not WaitForFirstConsumer)
Existing Kafka PVCs remain bound to old longhorn-kafka StorageClass (safe - no data loss).
New PVCs will use unified 'longhorn' StorageClass.