iommu/vt-d: Unnecessary to handle default identity domain

The iommu default domain framework has been designed to take
care of setting identity default domain type. It's unnecessary
to handle this again in the VT-d driver. Hence, remove it.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>