x86/smp: Remove a non-existent function declaration

x86_idle_thread_init() does not exist anywhere. Remove its declaration
from the header.
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230727180533.3119660-3-sohil.mehta@intel.com

arch/x86/include/asm/smp.h

@@ -132,7 +132,6 @@ void smp_kick_mwait_play_dead(void);
 void native_smp_send_reschedule(int cpu);
 void native_send_call_func_ipi(const struct cpumask *mask);
 void native_send_call_func_single_ipi(int cpu);
-void x86_idle_thread_init(unsigned int cpu, struct task_struct *idle);
 
 bool smp_park_other_cpus_in_init(void);