Purge set_defaults() from all the docs

[jgeewax]

Is set_defaults() fully not necessary anymore? If so, can we chop that from all the documentation?

Looks like they're in there quite a bit... (http://googlecloudplatform.github.io/gcloud-python/latest/datastore-api.html)

[dhermes]

Yes, we even discussed tearing it out as a method since set_default_connection() and set_default_dataset_id() exist.

[jgeewax]

Love it. I'll update the ticket to be more clear :)

[dhermes]

Tackling this right now.