sound: Retire pcm_feederdesc->type

This is always accessed from pcm_feeder->desc->type. Instead of
duplicating this field, we can remove it from pcm_feederdesc, and access
it through pcm_feeder->class->type.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53557