Add method for deleting data

[jsh2134]

Deleting data in a dataset requires a DatasetMigration in delete mode, which can be verbose and prone to syntax/arg mistakes.

We can simplify this with a .delete() method.

Current

>>> my_dataset.query().filter(some_value=True).migrate(my_dataset, commit_mode='delete')

Proposed

>>> my_dataset.query().filter(some_value=True).delete()