Remove download_url from setup.py

Github stopped providing download hosting, and it was
decided on the user ML that there's no need to set
download_url at all -- the normal pypi tarball will
suffice:
https://groups.google.com/forum/?fromgroups#!topic/pycassa-discuss/oQ_l_coZDFg

setup.py

@@ -118,7 +118,6 @@ def run(self):
       description = 'Python client library for Apache Cassandra',
       long_description = long_description,
       url = 'http://github.com/pycassa/pycassa',
-      download_url = 'http://github.com/pycassa/pycassa/archive/v%s.tar.gz' % __version__,
       keywords = 'cassandra client db distributed thrift',
       packages = ['pycassa',
                   'pycassa.cassandra',