REF: groupby remove _selection_name
We use self._selected_obj and self._obj_with_exclusions in different places with no clear distinction as to when to use which.  I _think_ in pretty much all cases we really want obj_with_exclusions, am moving towards pinning that down.

The only non-trivial thing here is getting rid of pinning _selection in [here](https://github.com/pandas-dev/pandas/compare/master...jbrockmendel:cln-selection_name?expand=1#diff-f1ec980d06b0b54c8263f663f767636c3f5921f04f8a7ee91dfc71a2100b05cdL326).  We have no tests that reach that.  I can cook up a case that gets there, but haven't found any cases where removing it actually changes anything.