[SPARK-12768][SQL] Remove CaseKeyWhen expression

This patch removes CaseKeyWhen expression and replaces it with a factory method that generates the equivalent CaseWhen. This reduces the amount of code we'd need to maintain in the future for both code generation and optimizer.

Note that we introduced CaseKeyWhen to avoid duplicate evaluations of the key. This is no longer a problem because we now have common subexpression elimination.

Author: Reynold Xin <rxin@databricks.com>

Closes #10722 from rxin/SPARK-12768.