powerpc/mm: Code cleanup for __hash_page_thp

This part was commented from commit 6d492ecc
("powerpc/THP: Add code to handle HPTE faults for hugepages")
in about 11 years before.

If there are no plans to enable this part code in the future,
we can remove this dead code and replace with a comment
explaining what the dead code was trying to say.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Suggested-by: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240301085834.1512921-1-chentao@kylinos.cn