Make AudioInputDialog and AudioOutputDialog use own icons.

Introduced config_audio_input.png and config_audio_output.png
for for that. Previously used config_basic.png.

Part of #1438
Drop unused files and icon.

Seems like we had some unreferenced files lying around.
- PAAudioConfig seems to be an (unfinished?) PulseAudio
  configuration dialog.
- PositionalSound seems to be an (unfinished?) positional
  audio configuration dialog.

They both used the config_dsound.png (directional? directsound?)
which this commit also drops as it doesn't seem to be used
anywhere.