powerpc/cell: Code cleanup for spufs_mfc_flush

This part was commented from commit a33a7d73


("[PATCH] spufs: implement mfc access for PPE-side DMA")
in about 18 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>
Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240126021258.574916-1-chentao@kylinos.cn