sound: Remove SND_DIAGNOSTIC section in buffer.c

The purpose of this has not been documented, but it seems like it makes
it possible to view the maximum number of bytes that passed to
sndbuf_feed(), as well as the maximum number of cycles taken inside its
main loop. These do not seem particularly useful anymore.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55989