Major Description
Once we implement Dataset-equivalent API in Python, we'd need to change the return type of map, flatMap, and mapPartitions. In this case, we should just remove them from Python DataFrame now in 2.0, so we don't break APIs in 2.x.
Users can still use those after the removal, but must prefix "rdd" to it. For example, df.rdd.map, df.rdd.flatMap, and df.rdd.mapPartitions.
