x86/nmi: Remove an unnecessary IS_ENABLED(CONFIG_SMP)

IS_ENABLED(CONFIG_SMP) is unnecessary here: smp_processor_id() should
always return zero on UP, and arch_cpu_is_offline() reduces to
!(cpu == 0), so this is a statically false condition on UP.

Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240201094604.3918141-1-xin@zytor.com