kernel: stathz is always non-zero after cpu_initclocks()

kernel: stathz is always non-zero after cpu_initclocks()

Now that the clockintr switch is complete, cpu_initclocks() always
initializes stathz to a non-zero value.  We don't call statclock()
from hardclock(9) anymore and, more broadly, we don't need to test
whether stathz is non-zero before using it.

With input from kettenis@.

Link: https://marc.info/?l=openbsd-tech&m=167434223309668&w=2
ok kettenis@ miod@