iommu/vt-d: Remove comment for def_domain_type

iommu/vt-d: Remove comment for def_domain_type

The comment for def_domain_type is outdated. Part of it is irrelevant.
Furthermore, it could just be deleted since the iommu_ops::def_domain_type
callback is properly documented in iommu.h, so individual implementations
shouldn't need to repeat that. Remove it to avoid confusion.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240624024327.234979-1-baolu.lu@linux.intel.com
Link: https://lore.kernel.org/r/20240702130839.108139-3-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>