[SPARK-17635][SQL] Remove hardcode "agg_plan" in HashAggregateExec

## What changes were proposed in this pull request?

"agg_plan" are hardcoded in HashAggregateExec, which have potential issue, so removing them.

## How was this patch tested?

existing tests.

Author: Yucai Yu <yucai.yu@intel.com>

Closes #15199 from yucai/agg_plan.