CLN: Removed dead GroupBy code

- [X] closes #20456
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20457?src=pr&el=h1) Report
> Merging [#20457](https://codecov.io/gh/pandas-dev/pandas/pull/20457?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/02477dab1e4dfb26597806899f8ebf2ca019c30a?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20457/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20457?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20457      +/-   ##
==========================================
+ Coverage    91.8%   91.82%   +0.01%     
==========================================
  Files         152      152              
  Lines       49223    49206      -17     
==========================================
- Hits        45191    45182       -9     
+ Misses       4032     4024       -8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <ø> (+0.01%)` | :arrow_up: |
| #single | `41.84% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20457?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20457/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.55% <ø> (+0.41%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20457/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.91% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20457?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20457?src=pr&el=footer). Last update [02477da...e07bb85](https://codecov.io/gh/pandas-dev/pandas/pull/20457?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!