sound: Remove outdated comment in dsp_oss_engineinfo()

Since we e07f9178502b ("sound: Separate implementations for
SNDCTL_AUDIOINFO[_EX] and SNDCTL_ENGINEINFO") support more than
mono/stereo.

Sponsored by: The FreeBSD Foundation
MFC after: 1 day
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D45605