x86: i8259A: remove redundant irq_descinitialization

Remove redundant irq_desc[NR_IRQS] element initialization in
init_ISA_irqs(). irq_desc[NR_IRQS] is already statically
initialized with the same values in kernel/irq/handle.c .

besides the clean-up value this also saves some space:

      text    data     bss     dec     hex filename
      1389     356      14    1759     6df i8259_32.o.before
      1325     356      14    1695     69f i8259_32.o.after

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>