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)