[SPARK-7485] [BUILD] Remove pyspark files from assembly.

The sbt part of the build is hacky; it basically tricks sbt
into generating the zip by using a generator, but returns
an empty list for the generated files so that nothing is
actually added to the assembly.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #6022 from vanzin/SPARK-7485 and squashes the following commits:

22c1e04 [Marcelo Vanzin] Remove unneeded code.
4893622 [Marcelo Vanzin] [SPARK-7485] [build] Remove pyspark files from assembly.