sound examples: Move MIDI example out of OSS directory

This example is not related to OSS.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, meka_tilda.center
Differential Revision: https://reviews.freebsd.org/D46308

(cherry picked from commit 6747b1a8218f229c7593cdb02e4b41e2b1186e26)