iommu/iommu-debug: Remove obsolete IOMMU domain attribute

The DOMAIN_ATTR_COHERENT_HTW_DISABLE IOMMU domain attribute is being
removed.  SMMU coherency will be configured through the SMMU device tree
nodes moving forward.  Remove the obsolete option.

Change-Id: If1251c945d71f8a7ab7c8e62c8a3d9d9139495ab
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>