sound: Remove unused dsp_cdevs[] fields and devices

These fields and devices are unused as of e8c0d15a64fa ("sound: Get rid
of snd_clone and use DEVFS_CDEVPRIV(9)").

While here, remove unused SND_DEV_* defines from pcm/sound.h and convert
the list to an enum.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D45013