[SPARK-13626][CORE] Revert change to SparkConf's constructor.

It shouldn't be private.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #11734 from vanzin/SPARK-13626-api.

core/src/main/scala/org/apache/spark/SparkConf.scala

@@ -47,7 +47,7 @@ import org.apache.spark.util.Utils
  *
  * @param loadDefaults whether to also load values from Java system properties
  */
-class SparkConf private[spark] (loadDefaults: Boolean) extends Cloneable with Logging {
+class SparkConf(loadDefaults: Boolean) extends Cloneable with Logging {
 
   import SparkConf._