irqstat: Get rid of nmi_count() and __IRQ_STAT()

Nothing uses this anymore.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20201113141733.005212732@linutronix.de