Remove reference to third-party drivers in readme
Remove reference to third-party drivers in readme

The README.rst mentions that third-party drivers are in the
neutron-fwaas tree.  This is no longer true as of the Newton release, so
remove that statement.

Change-Id: I1f9dc4c7b8ac4b2cd0f670e17b792e3670b96e09

openstack/neutron-fwaas,openstack/neutron-fwaas