[SPARK-11946][SQL] Audit pivot API for 1.6.

Currently pivot's signature looks like

```scala
scala.annotation.varargs
def pivot(pivotColumn: Column, values: Column*): GroupedData

scala.annotation.varargs
def pivot(pivotColumn: String, values: Any*): GroupedData
```

I think we can remove the one that takes "Column" types, since callers should always be passing in literals. It'd also be more clear if the values are not varargs, but rather Seq or java.util.List.

I also made similar changes for Python.

Author: Reynold Xin <rxin@databricks.com>

Closes #9929 from rxin/SPARK-11946.