irqchip/gic-v3: Remove gic_root_node variable from the ITS code

The gic_root_node variable defined in ITS driver is not actually
used, so just remove it.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>