REF: remove no-op casting
In _python_agg_general:

```
            if self.grouper._filter_empty_groups:
                mask = counts.ravel() > 0
```

It isn't entirely trivial, but we can show that we will always have `mask.all()`, as a result of which the remainder of this code chunk of code is a no-op.  This PR removes it.

As a consequence, we can remove `_filter_empty_groups` and `compressed`

nice!