It is not really used from any of the sources, including the
standalone app. Since we are moving to a more backend-independent
drawing it makes sense to remove header which was specific to
how Blender integrates Cycles into viewport.
There is probably some cleanup in CMake files is possible, but
there is some inter-dependency with USD.
Differential Revision: https://developer.blender.org/D16681
c5e71cebaa