WASAPI: remove irrelevant TODOs

The final volume conversion API has changed. In any case, we don't need
to convert the volume here; the Qt volume is linear and so is the
volume expected by IAudioStreamVolume.

Change-Id: I6065505735a6140843dcfefd8b2e0447fc83102d
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>