The goal here is to ensure we check for audio backends in a way that
makes sense. On macOS, let's just always use Audio Unit (and thus avoid
any checks for Pulse, to reduce needless/confusing build log noise). We
will also only use the Qt audio backend if no other backend was found,
rather than only checking for Pulse.
We had numerous NiH-based implementations of audio formats and metadata
that we now no longer need because we either don't make use of the code,
or we replaced its implementation by FFmpeg.
