etc/mtree: Remove entry for /usr/tests/usr.sbin/mixer

These were removed when the new mixer(3) library was imported, and I
missed updating the mtree file when I added the entries to
ObsoleteFiles.inc.

Fixes: 903873ce1560 Implement and use new mixer(3) library for FreeBSD.
(cherry picked from commit 7aa3bf6952b2c7d1b02d3a67e7f51b96fcf52b50)