[SPARK-20349][SQL][REVERT-BRANCH2.1] ListFunctions returns duplicate functions after using persistent functions

Revert the changes of https://github.com/apache/spark/pull/17646 made in Branch 2.1, because it breaks the build. It needs the parser interface, but SessionCatalog in branch 2.1 does not have it.

### What changes were proposed in this pull request?

The session catalog caches some persistent functions in the `FunctionRegistry`, so there can be duplicates. Our Catalog API `listFunctions` does not handle it.

It would be better if `SessionCatalog` API can de-duplciate the records, instead of doing it by each API caller. In `FunctionRegistry`, our functions are identified by the unquoted string. Thus, this PR is try to parse it using our parser interface and then de-duplicate the names.

### How was this patch tested?
Added test cases.

Author: Xiao Li <gatorsmile@gmail.com>

Closes #17661 from gatorsmile/compilationFix17646.