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