[SPARK-15278] [SQL] Remove experimental tag from Python DataFrame

## What changes were proposed in this pull request?
Earlier we removed experimental tag for Scala/Java DataFrames, but haven't done so for Python. This patch removes the experimental flag for Python and declares them stable.

## How was this patch tested?
N/A.

Author: Reynold Xin <rxin@databricks.com>

Closes #13062 from rxin/SPARK-15278.