[HOTFIX][SQL] Remove cleaning of UDFs

It is not safe to run the closure cleaner on slaves.  #2153 introduced this which broke all UDF execution on slaves.  Will re-add cleaning of UDF closures in a follow-up PR.

Author: Michael Armbrust <michael@databricks.com>

Closes #2174 from marmbrus/fixUdfs and squashes the following commits:

55406de [Michael Armbrust] [HOTFIX] Remove cleaning of UDFs