[SPARK-6046] Privatize SparkConf.translateConfKey

The warning of deprecated configs is actually done when the configs are set, not when they are get. As a result we don't need to explicitly call `translateConfKey` outside of `SparkConf` just to print the warning again in vain.

Author: Andrew Or <andrew@databricks.com>

Closes #4797 from andrewor14/warn-deprecated-config and squashes the following commits:

8fb43e6 [Andrew Or] Privatize SparkConf.translateConfKey