sched/hotplug: Move migration CPU_DYING to sched_cpu_dying()

Remove the hotplug notifier and make it an explicit state.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160310120025.502222097@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>