smpboot.h: Remove unused function prototype

Function smpboot_thread_schedule() is neither used nor defined,
so kill it.
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

include/linux/smpboot.h

@@ -47,6 +47,5 @@ struct smp_hotplug_thread {
 
 int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread);
 void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread);
-int smpboot_thread_schedule(void);
 
 #endif