cpu/hotplug: Remove redundant state check

The check for the AP range in cpuhp_is_ap_state() is redundant after commit
8df3e07e "cpu/hotplug: Let upcoming cpu bring itself fully up" because all
states above CPUHP_BRINGUP_CPU are invoked on the hotplugged cpu. Remove it.

Reported-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>