Remove WindowLevelInterface.tcl and WindowLevelInterface.py

This is a code fragment providing an interface to a viewer of
type vtkImageViewer. It seems to be just a helper used to setting the window
level etc. in the viewer.

I am proposing that this code be removed.

If there is a need to test Tkinter stuff then Attenuation.tcl from the
VTK Text Book exercises would be a good candidate. I have converted this to
VTK 6 and also written a Python version of it.

Marcus pointed out that Imaging/Core/Testing/Python/WindowLevelInterface.py
and has syntax errors and that this and the Tcl version are never run.

This arose from the VTK hackathon.

Change-Id: Ic57d8940a240e1939051f4e68d5a329bac645918
Signed-off-by: Andrew Maclean <andrew.amaclean@gmail.com>
Merge topic 'delete-WindowLevelInterface' into master

1bc76b6d Remove WindowLevelInterface.tcl and WindowLevelInterface.py