CLN: remove unused row_bool_subset
@jreback can you confirm my hunch that the `result.ndim == 2` block in groupby.ops was for Panel?

Tracing back the blame, row_bool_subset was introduced [here](https://github.com/pandas-dev/pandas/commit/b15ae85e1f647821755d35bedd6143ad1e9c3678) in 2012 to close #152 and it started being called only for result.ndim == 2 [here](https://github.com/pandas-dev/pandas/commit/9bfdc3c037e926143e708463d76fcdae3fb709f2)
Haven't looked deeply but seems like this is more for `ohlc` than Panel right? I speculate that it’s for Panel because if it is an agg op that should be a reduction, so we shouldn’t get a 2D result unless we started with 3D.  But “speculate” is definitely the important word in that sentence > so we shouldn’t get a 2D result unless we started with 3D

Yea that's not quite true with ohlc though. To illustrate:

```python
In [20]: ser = pd.Series([1], index=[pd.Timestamp('today')])

In [21]: ser.groupby([1]).ohlc()
Out[21]:
   open  high  low  close
1     1     1    1      1
```

Not sure what would specifically hit this branch but I think ohlc test coverage is weak so just need to be careful Dangit Will, you're gonna make me go and do actual critical thinking instead of mechanically reading the coverage results... Hmm that example doesn't break on this branch, so must not go through this code path.  Still, the point that test coverage is spotty is well-taken. thanks @jbrockmendel 

I don't really remember where this was used, does seem like its related to Panel though.