iommu/vt-d:Remove unused variable

Unused after commit <42e8c186> ("iommu/vt-d: Simplify io/tlb flushing
in intel_iommu_unmap"), cleanup it.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>