cpu/hotplug: Remove unused function declaration cpu_set_state_online()

Commit 5356297d ("cpu/hotplug: Remove cpu_report_state() and related unused cruft")
removed function but leave the declaration.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230805110406.45900-1-yuehaibing@huawei.com

include/linux/cpu.h

@@ -191,7 +191,6 @@ void arch_cpu_finalize_init(void);
 static inline void arch_cpu_finalize_init(void) { }
 #endif
 
-void cpu_set_state_online(int cpu);
 void play_idle_precise(u64 duration_ns, u64 latency_ns);
 
 static inline void play_idle(unsigned long duration_us)