iommu/amd: Remove unused amd_io_pgtable.pt_root variable

iommu/amd: Remove unused amd_io_pgtable.pt_root variable

It has been no longer used since the commit 6eedb59c18a3 ("iommu/amd:
Remove amd_iommu_domain_get_pgtable").

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20230921092147.5930-2-vasant.hegde@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>