Remove "unimplemented" warning in ofSoundShutdown

The function is called by the OF shutdown routine on all platforms, leading to log spam on non-FMOD platforms. [Brought up by @procedural here](https://github.com/openframeworks/openFrameworks/commit/981e4cfa6910618a96e48cd8a92b02175d509d68#commitcomment-8687571).