rc.subr(8): delete debug cpuset helper

Summary:
The intention of the original author (I assume) was to add this logic
for testing. This removes the debug statement so it no longer shows up
in calls to status.

MFC after: 2 weeks
MFC with: 0661f9389

Reviewers: kevans

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D40055