Cleanup of GroupBy Code

[WillAyd]

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

[codecov]

Codecov Report

Merging #26090 into master will decrease coverage by 51.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26090      +/-   ##
==========================================
- Coverage   91.96%   40.76%   -51.2%     
==========================================
  Files         175      175              
  Lines       52412    52419       +7     
==========================================
- Hits        48200    21369   -26831     
- Misses       4212    31050   +26838

Flag	Coverage Δ
#multiple	?
#single	40.76% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby/groupby.py	24.52% <ø> (-72.71%)	⬇️
pandas/core/groupby/generic.py	13.26% <ø> (-73.83%)	⬇️
pandas/io/formats/latex.py	0% <0%> (-100%)	⬇️
pandas/io/sas/sas_constants.py	0% <0%> (-100%)	⬇️
pandas/core/groupby/categorical.py	0% <0%> (-100%)	⬇️
pandas/tseries/plotting.py	0% <0%> (-100%)	⬇️
pandas/tseries/converter.py	0% <0%> (-100%)	⬇️
pandas/io/formats/html.py	0% <0%> (-99.37%)	⬇️
pandas/io/sas/sas7bdat.py	0% <0%> (-91.16%)	⬇️
pandas/io/sas/sas_xport.py	0% <0%> (-90.1%)	⬇️
... and 135 more

---

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[codecov]

Codecov Report

Merging #26090 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26090      +/-   ##
==========================================
+ Coverage   91.96%   91.98%   +0.02%     
==========================================
  Files         175      175              
  Lines       52412    52392      -20     
==========================================
- Hits        48200    48193       -7     
+ Misses       4212     4199      -13

Flag	Coverage Δ
#multiple	90.54% <ø> (+0.02%)	⬆️
#single	40.74% <ø> (-0.13%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby/generic.py	89.03% <ø> (+1.95%)	⬆️
pandas/io/gbq.py	75% <0%> (-12.5%)	⬇️
pandas/core/frame.py	96.9% <0%> (-0.12%)	⬇️

---

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[WillAyd]

Looks like the _assure_grouper method is actually subclassed by DateTimeIndexResample so added that back in for now

[jreback]

thanks!