[SPARK-21253][CORE] Disable spark.reducer.maxReqSizeShuffleToMem

## What changes were proposed in this pull request?

Disable spark.reducer.maxReqSizeShuffleToMem because it breaks the old shuffle service.

Credits to wangyum

Closes #18466

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>
Author: Yuming Wang <wgyumg@gmail.com>

Closes #18467 from zsxwing/SPARK-21253.