iommu/arm-smmu: Remove arm_smmu_flush_pgtable()

With the io-pgtable code now enforcing its own appropriate sync points,
the vestigial flush_pgtable callback becomes entirely redundant, so
remove it altogether.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>