[SPARK-13466] [SQL] Remove projects that become redundant after column pruning rule

JIRA: https://issues.apache.org/jira/browse/SPARK-13466

## What changes were proposed in this pull request?

With column pruning rule in optimizer, some Project operators will become redundant. We should remove these redundant Projects.

For an example query:

    val input = LocalRelation('key.int, 'value.string)

    val query =
      Project(Seq($"x.key", $"y.key"),
        Join(
          SubqueryAlias("x", input),
          BroadcastHint(SubqueryAlias("y", input)), Inner, None))

After the first run of column pruning, it would like:

    Project(Seq($"x.key", $"y.key"),
      Join(
        Project(Seq($"x.key"), SubqueryAlias("x", input)),
        Project(Seq($"y.key"),      <-- inserted by the rule
        BroadcastHint(SubqueryAlias("y", input))),
        Inner, None))

Actually we don't need the outside Project now. This patch will remove it:

    Join(
      Project(Seq($"x.key"), SubqueryAlias("x", input)),
      Project(Seq($"y.key"),
      BroadcastHint(SubqueryAlias("y", input))),
      Inner, None)

## How was the this patch tested?

Unit test is added into ColumnPruningSuite.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #11341 from viirya/remove-redundant-project.