Cleanup WindowExpr and AggregateExpr trait

[lewiszlw]

Which issue does this PR close?

Closes #.

Rationale for this change

Found some methods are never used in WindowExpr and AggregateExpr trait, as they are not public traits, so just remove them.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[berkaysynnada]

We use these methods in our fork. If there is no harm, we would prefer to keep them.

[lewiszlw]

We might eventually delete entire AggregateExpr trait (#11810).

[berkaysynnada]

I agree that AggregateExpr should be deleted, but I prefer to remove it at one shot. You are also removing more here.

[lewiszlw]

I could remove WindowExpr trait change in this pr.

Removing AggregateExpr trait at one shot will result a huge pr, it's hard for reviewing.

[ozankabak]

This would get the code in a state that breaks some downstream users (including us). Let's proceed one by one -- let's start off by focusing on removing AggregateExpr. We will help with reviewing.

If you want to do this in small PRs and make this the first step, you can start off by moving all_expressions and with_new_expressions APIs to AggregateFunctionExpr. Thanks

[lewiszlw]

Alright, I'll try to remove AggregateExpr trait in one pr.