powerpc/iommu: Code cleanup for cell/iommu.c

This part was commented from commit 165785e5

 ("[POWERPC] Cell
iommu support") in about 17 years before.

If there are no plans to enable this part code in the future,
we can remove this dead code.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240125082637.532826-1-chentao@kylinos.cn