[SPARK-13637][SQL] use more information to simplify the code in Expand builder

## What changes were proposed in this pull request?

The code in `Expand.apply` can be simplified by existing information:

* the `groupByExprs` parameter are all `Attribute`s
* the `child` parameter is a `Project` that append aliased group by expressions to its child's output

## How was this patch tested?

by existing tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11485 from cloud-fan/expand.