ARM: LPC32xx: Cleanup irq.c

This patch removes the IRQ mask initialization which is already done some lines
above.

This was actually a bug: The init was supposed to set the bits for the
(chained) SUB IRQs. But this is already fixed by the previous patch, doing this
implicitely via irq_set_chained_handler().

Signed-off-by: Roland Stigge <stigge@antcom.de>