[SPARK-16615][SQL] Expose sqlContext in SparkSession

## What changes were proposed in this pull request?
This patch removes the private[spark] qualifier for SparkSession.sqlContext, as discussed in http://apache-spark-developers-list.1001551.n3.nabble.com/Re-transtition-SQLContext-to-SparkSession-td18342.html

## How was this patch tested?
N/A - this is a visibility change.

Author: Reynold Xin <rxin@databricks.com>

Closes #14252 from rxin/SPARK-16615.