pcm: Turn SND_DECLARE_FILE into a no-op.

SND_DECLARE_FILE originally added lines to the output of /dev/sndstat
listing the $FreeBSD$ strings for individual files, but only if the
value of hw.snd.verbose was raised to 3. With the switch to Git these
strings became meaningless as they were now all identical and no longer
contained the path (which was implicitly included previously via the
keyword expansion).

This commit removes all of the infrastructure to support file version
strings from /dev/sndstat, but preserves the KPI/KBI by turning the
SND_DECLARE_FILE macro into a nop and changing the backing sysinit
functions into null functions and is suitable for merging to
stable/13.

A future commit will remove SND_DECLARE_FILE entirely.

Reviewed by: kbowling, emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D41498