COMP: Add VTKv6 Python path to launcher

Add the "standard" site-packages path to the launcher's list of
additional Python paths. This appears to be necessary for VTKv6 support,
as VTKv6 is installing its bindings here, and not to any location that
is in the old path list, and determining the Python version in order to
override that location isn't obvious/trivial. (Plus, the "new" location
conforms better with - if not FHS, technically - the conventional
locations for such things, which is something we should be working
toward anyway.)

Also, remove specification of VTK_INSTALL_PYTHON_USING_CMAKE, which no
longer exists in VTKv6.