iommu/amd: Remove amd_iommu_domain_get_pgtable

Since the IO page table root and mode parameters have been moved into
the struct amd_io_pg, the function is no longer needed. Therefore,
remove it along with the struct domain_pgtable.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20201215073705.123786-9-suravee.suthikulpanit@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>