Remove use of old CMakeBackwardCompatibilityC variables:

 CMAKE_USE_PTHREADS
 CMAKE_USE_SPROC
 CMAKE_HP_PTHREADS
 CMAKE_USE_WIN32_THREADS
 CMAKE_NO_STD_NAMESPACE
 CMAKE_ANSI_FOR_SCOPE

Use of these was originally removed by

 commit fe401e0f (Remove old CMakeBackwardCompatibilityC thread options, 2012-02-23)
 commit e56a7eb9 (Drop workaround for no std:: namespace, 2012-02-23)
 commit ff74963a (Drop workaround for non-ANSI 'for' scope, 2012-02-23)

but accidentally restored by modularization.  Drop use of the variables
again.  The thread variables in particular interfere with the thread
library selection done by the top-level CMakeLists.txt file.

While at it, drop unused computation of VTK_USE_OPENGL_LIBRARY from
Common/Core.  Its value is hard-coded in Rendering/OpenGL when the
module is enabled.

Change-Id: Ife7730fbfe32c51f0c431d440935636b2bb34c39