irqchip/gic-v3-its: Remove BUG_ON in its_vpe_irq_domain_alloc

This BUG_ON() is useless, because the same effect will be obtained 
by letting the code run its course and vm being dereferenced,
triggering an exception.

So just remove this check.

Signed-off-by: Guanrui Huang <guanrui.huang@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240418061053.96803-3-guanrui.huang@linux.alibaba.com