sound: Remove PCM_DETACHING(), SD_F_DETACHING and SD_F_DYING

Since SD_F_REGISTERED is cleared at the same time SD_F_DETACHING and
SD_F_DYING are set, and since PCM_DETACHING() is always used in
conjuction with PCM_REGISTERED()/DSP_REGISTERED(), it is enough to just
check SD_F_REGISTERED.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47463

(cherry picked from commit 6d4c59e26189a8c19fd0832e89f9b089330cbfcb)