BUG: ITKPython: Add PYTHONPATH support to the launcher for ITK build tree.

This commit removes pythonpath detection script that was only
working when Slicer_USE_ITKPython was enabled with an existing
build tree. Indeed, when doing a clean build, it was failing
because Slicer python interpreter was not available at configure time.

To address this, this commit configures the PYTHONPATH launcher
settings for the build tree.

Co-authored-by: Johan Andruejol <johan.andruejol@kitware.com>
Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionR@kitware.com>

From: Matt McCormick <matt.mccormick@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24892 3bd1e089-480b-0410-8dfb-8563597acbee