Remove legacy OpenGL 2.1 renderer

audio/audiorenderer.h

@@ -30,7 +30,6 @@ http://mozilla.org/MPL/2.0/.
 
 #define EU07_SOUND_PROOFINGUSESRANGE
 
-class opengl_renderer;
 class sound_source;
 
 using uint32_sequence = std::vector<std::uint32_t>;
@@ -101,8 +100,6 @@ private:
 
 class openal_renderer {
 
-    friend opengl_renderer;
-
 public:
     openal_renderer() = default;
     ~openal_renderer();