Simplify AggregateHashTable<PartialSkip> #23113

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• [EPIC] Split Aggregation Logic into Dedicated Streams #22710

In the above EPIC's aggregate refactor, it's now using a variant of AggregateHashTable<ParitalSkip> to handle the convert_to_states() optimization (see #22899 for this optimization)
https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/aggregates/hash_table.rs#L601

The only needed fields are only GroupsAccumulators, and we don't have to carry along the heavy hash table, so we can simplify AggregateHashTable<PartialSkip> into a simpler struct.

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