genirq: Do not fiddle with IRQ_MASKED in handle_edge_irq()

IRQ_MASKED is set in mask_ack_irq() anyway. Remove it from
handle_edge_irq() to allow simpler ab^HHreuse of that function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20110202212551.918484270@linutronix.de>