iommu/vt-d: Remove unused variable

Unused after commit 71684406 ("iommu/vt-d: Detach domain *only*
from attached iommus").  Reported by 0-day builder.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>