sound: Retire MAXFEEDERS and feedercnt

Apart from the fact that it's unrealistic to reach MAXFEEDERS (256),
since sound(4) comes with maximum 7 feeders (including feeder_root),
there is also no reason to cap it.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53549

(cherry picked from commit 84282785cc497c3fb798502899f880e15d063233)