datastore: remove support for gRPC call options
datastore: remove support for gRPC call options

This was added as an attempt to address a specific user problem. That
problem has been resolved.

We may add such a feature in the future, but in a different way.

Change-Id: Id5ada6c35d8692045fd5c26d3554ef4ea0976a57
Reviewed-on: https://code-review.googlesource.com/8890
Reviewed-by: Jaana Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go