{audio,sysutils}/Makefile: Remove hooks of deleted ports
PR: 296049
Reported by: chris.torek@gmail.com
Fixes: 2717347b94f9 ("audio/freebsd-13-mixer: Discontinue")
Fixes: c3c20bddefe4 ("sysutils/mixer: Discontinue")
Approved by: osa, vvd (Mentors, implicit)