mm: Remove arch_remap() and mm-arch-hooks.h

powerpc was the last provider of arch_remap() and the last
user of mm-arch-hooks.h.

Since commit 526a9c4a ("powerpc/vdso: Provide vdso_remap()"),
arch_remap() hence mm-arch-hooks.h are not used anymore.

Remove them.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Richard Weinberger <richard@nod.at>