[SPARK-19148][SQL][FOLLOW-UP] do not expose the external table concept in Catalog

### What changes were proposed in this pull request?
After we renames `Catalog`.`createExternalTable` to `createTable` in the PR: https://github.com/apache/spark/pull/16528, we also need to deprecate the corresponding functions in `SQLContext`.

### How was this patch tested?
N/A

Author: Xiao Li <gatorsmile@gmail.com>

Closes #17502 from gatorsmile/deprecateCreateExternalTable.