sound: Retire SD_F_AUTOVCHAN

This flag is redundant and essentially a no-op, as it is set when the
device supports at least playback or recording, which is almost always
the case. But even if the device is mixer-only (i.e., 0 channels), there
is no reason to keep this flag; it is only used to bail out of the vchan
sysctl handlers, but we already bail out anyway if we try to use the
sysctl in a vchan direction that is not supported.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49021