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GH-43986: [C++][Acero] Some code cleanup to `Grouper` (#43988)



### Rationale for this change

See #43986.

### What changes are included in this PR?

Mostly trivial changes, plus removing one `Grouper` implementation that's not wired.

### Are these changes tested?

No new tests needed.

### Are there any user-facing changes?

None.

* GitHub Issue: #43986

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>