ia64: Remove unused check_irq_used()

Just stumbled over it when staring into ia64 irq handling.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Tony Luck <tony.luck@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/20140507154336.566531793@linutronix.de


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