parisc: Remove bogus __IRQ_STAT macro

This is a leftover from a historical array based implementation and unused.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20201113141732.680780121@linutronix.de