cpu/hotplug: Remove export of cpu_active_mask and cpu_dying_mask

No module users and no module should ever care.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
[Dennis: applied cleanly]
Signed-off-by: Dennis Zhou <dennis@kernel.org>