iommu/vt-d: Remove unused variable

Unused after commit 71684406905f ("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>

drivers/iommu/intel-iommu.c

@@ -1742,7 +1742,6 @@ static int domain_init(struct dmar_domain *domain, int guest_width)
 
 static void domain_exit(struct dmar_domain *domain)
 {
-	struct dmar_drhd_unit *drhd;
 	struct page *freelist = NULL;
 	int i;