Feature Request: Remove management annotations from empty implicit namespaces

### Checklist

- [x] I did not find a related open enhancement request.
- [x] I understand that enhancement requests filed in the GitHub repository are by default low priority.
- [x] If this request is time-sensitive, I have submitted a corresponding issue with [GCP support](https://cloud.google.com/support-hub).

### Describe the feature

Description: If a user removes the last managed resource from an [implicit namespace](https://www.google.com/search?q=https://cloud.google.com/anthos-config-management/docs/how-to/namespace-resolution%23implicit-namespaces) in their Git repository, Config Sync leaves the management labels and annotations on the Namespace object in the cluster. This creates a stale state where the Namespace appears to be managed by Config Sync, even though no resources within it are currently being synced.

Feature request: We request that the multi-repo reconciler be updated to detect when an implicit namespace no longer contains any managed resources. When this occurs, it should automatically strip the management annotations and labels from the Namespace, accurately reflecting that it is no longer under active management for that repository.

Alternatives considered:
- Deleting the Namespace entirely (disruptive if the Namespace contains other unmanaged resources).

### Importance

_No response_