REF: remove unnecessary exception in groupby
in a previous pass we made this unnecessary be pre-checking for the condition under which a ValueError was raised.  This removes the try/except.

cc @jreback orthonal to other groupby PRs
@gfyoung 

the fact that this passes current groupby tests is good enough for the refactoring here. I think @jbrockmendel is trying to simplify this code overall.