sound: Remove unused FEEDER_DEBUG
No functional change intended.
Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D45984
(cherry picked from commit c55f995502cbaa2d38b7a7003bc27805b28a6a29)