sound: Make CHN_REMOVE_SAFE() the default

Commit 27b932e32fab ("sound: Safely remove channel from list in one
pass") optimized CHN_REMOVE_SAFE() to perform almost equally to
CHN_REMOVE(), so we can turn CHN_REMOVE_SAFE() into CHN_REMOVE() without
fears of performance regressions, while also being more robust.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D48249

(cherry picked from commit ffcefe5310e084415a2f292a00f4637d4059c40f)