Remove the getStageInfo() method from SparkContext.

This method exposes the Stage objects, which are
private to Spark and should not be exposed to the
user.

This method was added in https://github.com/apache/spark/commit/01d77f329f5878b7c8672bbdc1859f3ca95d759d; ccing @squito here in case there's a good reason to keep this!