[SQL][minor] Removed unused Catalyst logical plan DSL.

The Catalyst DSL is no longer used as a public facing API. This pull request removes the UDF and writeToFile feature from it since they are not used in unit tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #6350 from rxin/unused-logical-dsl and squashes the following commits:

90b3de6 [Reynold Xin] [SQL][minor] Removed unused Catalyst logical plan DSL.