This question came up in the review in #282 while trying to define the "correct" behavior of datastore.Key.__eq__.

The only remaining use of Key._dataset_id is in to_protobuf but #121 seems to indicate that the dataset ID is not needed on a Key.

ISTM we should just remove _dataset_id from the Key class, even though it is returned in the protobuf after an entity is stored/retrieved. @pcostell WDYT?