ao_coreaudio_exclusive: rip out pseudo volume control

This could mute a digital passthrough stream by writing zeros. All other
volume values did nothing.

The comment about MPlayer dying hasn't been true in mpv for quite a
while. It's even possible that it's fixed in upstream MPlayer. mpv will
print a scary error message when trying to change volume with spdif, and
continue normally.

If we really want to mute by writing zeros, we should do it in a
separate filter. But I'm not overly fascinated by this approach; is it
even guaranteed receivers will not be confused by a stream of zeros?

The main reason to remove this is that it's in the way of further
cleanups.

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