callout: Simplify cpuid validation in callout_reset_sbt_on()

• Remove a flag variable.
• Convert a runtime check of the passed cpuid to a KASSERT.
• Remove the cc_inited flag. An attempt to schedule a callout before SI_SUB_CPU will crash anyway since the per-CPU mutexes won't have been initialized, and that flag was only checked in the case where a cpuid was explicitly specified by the caller.

No functional change intended.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation