ARM: lpc32xx: fix NR_IRQS confict

With the change to sparse IRQs, the lpc32xx platform gets a warning about
conflicting macros:

In file included from arch/arm/mach-lpc32xx/irq.c:31:0:
arch/arm/mach-lpc32xx/include/mach/irqs.h:115:0: warning: "NR_IRQS" redefined
 #define NR_IRQS    96
arch/arm/include/asm/irq.h:9:0: note: this is the location of the previous definition
 #define NR_IRQS NR_IRQS_LEGACY

One such instance was in the old irq driver that is now removed by
the previous patch, but any other file including mach/irqs.h still
has the issue. Since none of them use this constant, we can just
remove the old definition.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 8cb17b5e ("irqchip: Add LPC32xx interrupt controller driver")