Code Clean Up - groupby_helper.pxi.in
A few of the methods in `groupby_helper.pxi.in` have a pattern that goes:

```python

    N, K = (<object> values).shape

    with nogil:

        if K > 1:
            # loop over j in range(K) and do stuff
            ...
        else:
            # do stuff with j=0
            ...
```

Unless I'm missing something simple I don't think there's a reason to have the conditional and duplicate code branches. In the case where K=1 (which is all the time?) then the `for j in range(K)` construct inherently sets `j=0`, bypassing the need for the conditional