s390: Remove pointless arch_show_interrupts()

S390 is not using the generic show interrupts implementation so the
extra arch_show_interrupts() is just useless.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux390@de.ibm.com
Link: http://lkml.kernel.org/r/20140507154339.609149081@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>