irqdomain: Remove unnecessary test for IRQ_DOMAIN_MAP_LEGACY

Where irq_domain_associate() is called in irq_create_mapping, there is
no need to test for IRQ_DOMAIN_MAP_LEGACY because it is already tested
for earlier in the routine.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rob Herring <rob.herring@calxeda.com>