iommu/vt-d: Remove device_to_iommu() call from domain_remove_dev_info()

This was problematic because it works by domain/bus/devfn and we want
to make device_to_iommu() use only a struct device * (for handling non-PCI
devices). Now that the iommu pointer is reliably stored in the
device_domain_info, we don't need to look it up.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>