x86: remove nesting CONFIG_HOTPLUG_CPU

prefill_possible_map() is defined inside CONFIG_HOTPLUG_CPU,
so the nesting CONFIG_HOTPLUG_CPU is just redundant.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>