Remove no longer needed class DatanodeAdminNodeDetails

DatanodeAdminNodeDetails was added earlier in the decommission branch, to track metrics and, the decommission state and maintenance end time.

After enhancing NodeStatus to old the Maintenance Expiry time, this class is no longer needed and it also duplicates information which is stored in other existing places.

This change removes it and then metrics etc can be added later in a different way.