remove Graphics/BackendGraphics.cpp

This file is never used and that might explain why it contains code that
clearly is not supposed to be there. I guess that i copied the file from
Renderer.cpp and never included it in the build. the BackendGraphics
class only has pure virtual methods anyway.
LINUX: g++ complains that NULL is not defined.

Microsoft's compiler defines NULL without the need to include stddef.h
If NULL should be used, one should make sure to #include <cstddef>

However in this case, the expression containing NULL can be rewritten.