msm: ipa: 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: I35c9e8ac92cd2e69ff6980f5314eebb0f74a6d46
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>