ARM: shmobile: sh73a0: Remove unneeded nr_irqs initialization

As per arch_probe_nr_irqs(), the default value is NR_IRQS, which maps to
NR_IRQS_LEGACY if CONFIG_SPARSE_IRQ=y.

Since SPARSE_IRQ is selected by both ARCH_MULTIPLATFORM and
ARCH_SHMOBILE_LEGACY, it's always enabled on shmobile.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>