AUDIO: Don't delete soundfont before cleaning up

This is useless as it will get destroyed by delete_fluid_synth and it
causes troubles with the Fluidsynth embedded OSAL which does not support
threads.
Indeed, deleting a soundfont in use relies on a timer thread to do the
cleanup after it's been released.

audio/softsynth/fluidsynth.cpp

@@ -517,9 +517,6 @@ void MidiDriver_FluidSynth::close() {
 
 	_mixer->stopHandle(_mixerSoundHandle);
 
-	if (_soundFont != -1)
-		fluid_synth_sfunload(_synth, _soundFont, 1);
-
 	delete_fluid_synth(_synth);
 	delete_fluid_settings(_settings);
 }