sound: Deprecate 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
MFC after: 1 day
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D50606

(cherry picked from commit 799105f53a9647f86dbf08456260d5850851b6f9)