[SPARK-17107][SQL][FOLLOW-UP] Remove redundant pushdown rule for Union

## What changes were proposed in this pull request?
Also remove useless function `partitionByDeterministic` after the changes of https://github.com/apache/spark/pull/14687

## How was this patch tested?
N/A

Author: gatorsmile <gatorsmile@gmail.com>

Closes #19097 from gatorsmile/followupSPARK-17107.