net: cnss: 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: I99bfca30b073ba3e2fed5d0cae87f2cc1d0914c4
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>