smpboot: Remove leftover declaration

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

kernel/smpboot.h

@@ -3,8 +3,6 @@
 
 struct task_struct;
 
-int smpboot_prepare(unsigned int cpu);
-
 #ifdef CONFIG_GENERIC_SMP_IDLE_THREAD
 struct task_struct *idle_thread_get(unsigned int cpu);
 void idle_thread_set_boot_cpu(void);