iommu/arm-smmu-v3: Remove ARM_SMMU_DOMAIN_NESTED

Currently this is exactly the same as ARM_SMMU_DOMAIN_S2, so just remove
it. The ongoing work to add nesting support through iommufd will do
something a little different.

Reviewed-by: Moritz Fischer <mdf@kernel.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Will Deacon <will@kernel.org>