sched_cpu_init: remove unnecessary NULL-checks for clockintr pointers

sched_cpu_init: remove unnecessary NULL-checks for clockintr pointers

sched_cpu_init() is only run once per cpu_info struct, so we don't
need these NULL-checks.

The NULL-checks are a vestige of clockintr_cpu_init(), which runs more
than once per CPU and uses the checks to avoid leaking clockintr handles.

Thread: https://marc.info/?l=openbsd-tech&m=169349579804340&w=2

ok claudio@