icnss: 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: I1ec7126144c41fa4215405e632d1d7b61de057bc
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>