sound: Retire SND_OLDSTEREO

This is a legacy option and does not serve a good purpose anymore.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55933

(cherry picked from commit bf0e5d987ecd2cbec2200e100bad6d6e724e9315)