Clean-up: Remove AggregateExec::group_by()

[berkaysynnada]

Which issue does this PR close?

Closes #.

Rationale for this change

AggregateExec already has

pub fn group_expr(&self) -> &PhysicalGroupBy {
     &self.group_by
 }

No need for

 pub fn group_by(&self) -> &PhysicalGroupBy {
     &self.group_by
 }

What changes are included in this PR?

AggregateExec:: group_by() is removed.

Are these changes tested?

Yes, existing tests.

Are there any user-facing changes?

A public function is removed, but it has an existing duplicate.

[ozankabak]

Thanks for the cleanup. @alamb, any reason to hold on to this cruft that I may be overlooking?

[alamb]

Looks good to me -- thank you 🙏