iommu: Remove unused IOMMU_SYS_CACHE_ONLY flag

The IOMMU_SYS_CACHE_ONLY flag was never exposed via the DMA API and
has no in-tree users. Remove it.

Cc: Robin Murphy <robin.murphy@arm.com>
Cc: "Isaac J. Manjarres" <isaacm@codeaurora.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Will Deacon <will@kernel.org>