irqchip/gic-v3-its: Remove unnecessary oom message

Fixes scripts/checkpatch.pl warning:
WARNING: Possible unnecessary 'out of memory' message

Remove it can help us save a bit of memory.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210609140643.14531-1-thunder.leizhen@huawei.com