irqchip/gic-v3-its: Remove superfluous WARN_ON

"ITS virtual pending table not cleaning" is already complained inside
its_clear_vpend_valid(), there's no need to trigger a WARN_ON again.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200206075711.1275-6-yuzenghui@huawei.com