ObsoleteFiles.inc: Remove old mixer(8) tests

These were removed when the new mixer(3) library was imported.

PR: 274602
Fixes: 903873ce1560 Implement and use new mixer(3) library for FreeBSD.

(cherry picked from commit f7c4f49649717000238184a975b2339bcd8c06d9)
(cherry picked from commit e8daee32125301676fb246671532abffbdca3e77)

Approved by: re (gjb)