Cleanup aggregation planning

### Is your feature request related to a problem or challenge?

part of #22710

Now the aggregation planning is implemented with a list of rules:
- https://github.com/apache/datafusion/blob/bbb552f71c93cdfadf91b374a3c4ff5f1a931ae7/datafusion/physical-plan/src/aggregates/mod.rs#L1189

After the refactor in the EPIC is done, we could probably clean them up into a simpler form.

A related idea is to add a better top-level specification for aggregation planning, to better describe this code. Currently, the comment at the top of the file explains the planning logic, but only at the level of intuition and high-level ideas.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_