sound: Retire SND_MULTICHANNEL
SND_MULTICHANNEL is always defined, so SND_CHN_MAX will also always be
- Apart from the fact that there is no other place in the code that
touches SND_MULTICHANNEL, there is also no good reason nowadays to set
SND_CHN_MAX to 2.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55934