[SPARK-17757][SQL] Remove CreateNamedStruct[Unsafe]

[hvanhovell]

What changes were proposed in this pull request?

This PR removes the CreateNamedStruct and CreateNamedStructUnsafe expressions. This only for simplification purposes.

How was this patch tested?

Existing tests.

[SparkQA]

Test build #66233 has finished for PR 15323 at commit 2e80037.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #66247 has finished for PR 15323 at commit 71eac80.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #66254 has finished for PR 15323 at commit 1dcca2e.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • trait CreateStructLike extends Expression
  • case class CreateStruct(children: Seq[Expression], names: Seq[String]) extends CreateStructLike
  • case class CreateStructUnsafe(

[hvanhovell]

closing this in favor of https://github.com/apache/spark/pull/14444/files