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

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 526a9c4a7234 ("powerpc/vdso: Provide vdso_remap()"),
arch_remap() hence mm-arch-hooks.h are not used anymore.

Remove them.

Link: https://lkml.kernel.org/r/320d7a9ed7b379a6e0edf16d539bc22447272e65.1608299993.git.christophe.leroy@csgroup.eu
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>