We added a bunch of higher order statistics such as skewness and kurtosis to GroupedData. I don't think they are common enough to justify being listed, since users can always use the normal statistics aggregate functions.

That is to say, after this change, we won't support
```scala
df.groupBy("key").kurtosis("colA", "colB")
```

However, we will still support
```scala
df.groupBy("key").agg(kurtosis(col("colA")), kurtosis(col("colB")))
```

Author: Reynold Xin <rxin@databricks.com>

Closes #9446 from rxin/SPARK-11489.