Revert "datastore: derive GCP project ID from GOOGLE_CLOUD_PROJECT."
Revert "datastore: derive GCP project ID from GOOGLE_CLOUD_PROJECT."

This reverts commit 9325a156d14be1f6560533a043f11754ecaf3f8f.

Reason for revert: Concerns raised by cbro@. Will follow up to make necessary changes.

Change-Id: I12f31f450c7535bd06aee9ca44105a51bc4cf92f
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/54131
Reviewed-by: Chris Cotter <b879ae7d4b52f11ce9f60d9854075eb8eebaf7a9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go