[SPARK-13635] [SQL] Enable LimitPushdown optimizer rule because we have...

[SPARK-13635] [SQL] Enable LimitPushdown optimizer rule because we have whole-stage codegen for Limit

JIRA: https://issues.apache.org/jira/browse/SPARK-13635

## What changes were proposed in this pull request?

LimitPushdown optimizer rule has been disabled due to no whole-stage codegen for Limit. As we have whole-stage codegen for Limit now, we should enable it.

## How was this patch tested?

As we only re-enable LimitPushdown optimizer rule, no need to add new tests for it.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #11483 from viirya/enable-limitpushdown.