CMake: Don't search for the OpenGL library on Mac OS X. It's always there.

CMakeLists.txt

@@ -1151,10 +1151,6 @@ elseif(APPLE)
       set(SDL_VIDEO_OPENGL 1)
       set(SDL_VIDEO_OPENGL_CGL 1)
       set(SDL_VIDEO_RENDER_OGL 1)
-      if(DARWIN)
-        find_library(OpenGL_LIBRARY OpenGL)
-        list(APPEND EXTRA_LIBRARIES ${OpenGL_LIBRARY})
-      endif()
       set(HAVE_VIDEO_OPENGL TRUE)
     endif()
   endif()