iommu: dma-mapping-fast: Remove unused function

Remove fast_smmu_release_mapping as it is no longer used.
iova_cookie lifetime is tied to the iommu_domain lifetime
instead.

Change-Id: Ib0b7055e1f2b953f90015d4e9d370de2c03a18e0
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>