sound: Retire the MIDI sequencer

The in-kernel MIDI sequencer is not used anymore, since this is done by
userland applications nowadays. It also contains bugs, and we are not
exactly sure how it works, or if it worked properly in the first place.

Sponsored by: The FreeBSD Foundation
Reviewed by: vishwin, markj
Differential Revision: https://reviews.freebsd.org/D50605