iommufd/selftest: Drop parent domain from mock_iommu_domain_nested

iommufd/selftest: Drop parent domain from mock_iommu_domain_nested

There is no use of this parent domain. Delete the dead code.

Note that the s2_parent in struct mock_viommu will be a deadcode too. Yet,
keep it because it will be soon used by HW queue objects, i.e. no point in
adding it back and forth in such a short window. Besides, keeping it could
cover the majority of vIOMMU use cases where a driver-level structure will
be larger in size than the core structure.

Link: https://patch.msgid.link/r/0f155a7cd71034a498448fe4828fb4aaacdabf95.1749882255.git.nicolinc@nvidia.com
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>