MacGui: get rid of now obsolete @"UseCoreAudio" preference and related widgets

MacGui doesn't include faac anymore. Mapping from faac to Core Audio is done by libhb's fallback mechanism.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5594 b64f7644-9d1e-0410-96f1-a4d463321fa5