x86/apic: Remove unused extern declarations

The removal of get_physical_broadcast(), generic_bigsmp_probe() and
default_apic_id_valid() left the stale declarations around.

Remove them.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240810093610.2586665-1-yuehaibing@huawei.com