microblaze: Remove stale irq_chip.end

irq_chip.end got obsolete with the removal of __do_IRQ().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Tested-by: Michal Simek <monstr@monstr.eu>
LKML-Reference: <20110203004210.240154507@linutronix.de>