Cleanup of GroupBy Code
Removing some dead code I came across on review of the module which appears in NDFrameGroupBy but is overridden by its only public child class DataFrameGroupBy

FWIW with the removal of PanelGroupBy the NDFrameGroupBy is an unnecessary layer of inheritance and could probably be merged in with DataFrameGroupBy, though I figure that could be a separate dedicated PR to merge those if so desired