Add method for deleting data

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()
```

