x86/apic: Remove declaration of unused hw_nmi_is_cpu_stuck

Commit 10f90149

 ("x86: Cleanup hw_nmi.c cruft") removed unused
code in the hw_nmi.c file because of the redesign of the hardlockup
watchdog but left declaration of hw_nmi_is_cpu_stuck in linux/nmi.h,
so remvoe it.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Link: http://lkml.kernel.org/r/1458697210-3027-1-git-send-email-baiyaowei@cmss.chinamobile.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>