sound: Retire /dev/midistat

This doesn't register all MIDI devices, for example USB ones, and is not
widely used in the first place. In the future all MIDI devices will
register to sndstat, where we already have defined a MIDI device type,
but we haven't made use of it yet.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50610