[SPARK-18666][WEB UI] Remove the codes checking deprecated config spark.sql.unsafe.enabled

## What changes were proposed in this pull request?

`spark.sql.unsafe.enabled` is deprecated since 1.6. There still are codes in UI to check it. We should remove it and clean the codes.

## How was this patch tested?

Changes to related existing unit test.

Please review http://spark.apache.org/contributing.html before opening a pull request.

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

Closes #16095 from viirya/remove-deprecated-config-code.