iommu: arm-smmu: Remove duplicate tlb_sync call

TLB sync is already being done in arm_smmu_tlb_inv_context_s1,
which is called at the end of all unmap calls.

Change-Id: I89929187a96816eecdd45d380af3a69515105b3e
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>