sound: Remove early SD_F_SIMPLEX and SD_F_AUTOVCHAN assignments

These flags are properly set in pcm_setstatus(), once the primary
channels have been created. The existing comment already states that
this is wrong.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47324

(cherry picked from commit 3a7d40c692622cc614a3839491c345d945f474fe)