kernel: disable hardclock() on secondary CPUs

kernel: disable hardclock() on secondary CPUs

There is no useful work left for secondary CPUs to do in hardclock().
Disable cq_hardclock on secondary CPUs and remove the now-unnecessary
early-return from hardclock().

This change reduces every system's normal clock interrupt rate by
(HZ - HZ/10) per secondary CPU.  For example, an 8-core machine
with a HZ=100 kernel should see its clock interrupt rate drop from
~1600 to ~960.

Thread: https://marc.info/?l=openbsd-tech&m=170750140915898&w=2

ok kettenis@