sound: Do not create root feeder in chn_init()

The feeder chain gets destroyed when feeder_chain() is called, which is
after the chn_reset() call in chn_init() further down for primary chans,
or vchan_create() for vchans. This makes the root feeder creation in
chn_init() essentially a no-op. Remove it altogether and let
feeder_chain() after chn_reset() take care of creating the feeder
properly. It creates the root one as well.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55941