[SPARK-19495][SQL] Make SQLConf slightly more extensible - addendum

## What changes were proposed in this pull request?
This is a tiny addendum to SPARK-19495 to remove the private visibility for copy, which is the only package private method in the entire file.

## How was this patch tested?
N/A - no semantic change.

Author: Reynold Xin <rxin@databricks.com>

Closes #17555 from rxin/SPARK-19495-2.