audio_core: Remove temp_mix_buffer

It's unused and doesn't need to be initialized

src/audio_core/audio_renderer.h

@@ -73,7 +73,6 @@ private:
     Core::Memory::Memory& memory;
     CommandGenerator command_generator;
     std::size_t elapsed_frame_count{};
-    std::vector<s32> temp_mix_buffer{};
 };
 
 } // namespace AudioCore