powerpc/powernv/ioda: Remove obsolete iommu_table_ops::exchange callbacks

As now we have xchg_no_kill/tce_kill, these are not used anymore so
remove them.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190829085252.72370-6-aik@ozlabs.ru