Use client.delete_dataset on BigQuery to drop datasets

[drewbanin]

Describe the feature

dbt should use the client.delete_dataset method to atomically drop BigQuery datasets:

client.delete_dataset(dataset_id, delete_contents=True, not_found_ok=True)

This is an improvement over the existing implementation which first deletes all of the tables in the dataset, then deletes the empty dataset. Some users have reported dbt failing to delete some of the tables in a dataset (and not removing the dataset itself), and I suspect that this change will fix that issue. At the very least, the client.delete_dataset implementation is simpler than the existing implementation.