Partial2Mode is not necessary in hash aggregation

Feature Request

Is your feature request related to a problem? Please describe:

hash aggregation with Coprocessor fetches partial results from storage and runs

1. Partial2Mode, partial results --> partial results
2. FinalMode, partial results --> final results

It seems that Partial2Mode is a redundant run, we can remove it for better code and performance.

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Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: