msm: ADSPRPC: 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: I8bf9b5115db0394620fa96f4bdaf0faec4528709
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>