Drop access to native window.

The new NFD library is not using this information, and the old one was using
it only on Windows. Oh well.

Removal of this functionality also removes some build-time decisions.

profiler/src/Backend.hpp

@@ -24,7 +24,6 @@ public:
     void SetTitle( const char* title );
 
     float GetDpiScale();
-    void* GetNativeWindow();
 
 private:
     WindowPosition m_winPos;