Clean up some of the CMake configuration for Tk.  For example, no
Tk configuration belongs in Common/Core anymore.  TK configuration
has been moved to ThirdParty/TclTk, which is now a dependency of
both Rendering/Tk and Wrapping/Tcl.

Remove vtkCommonCoreTCL from vtkRenderingPythonTkWidgets link, the
Python/Tk connection does not require the Tcl wrappers at all.

Change-Id: Ib869e6d89e7ea04294e1ab271aec3319d033a9fc