xen/arm: mm: Remove unused M2P code

xen/arm: mm: Remove unused M2P code

Arm does not have an M2P and very unlikely to get one in the future,
therefore don't keep defines that are not necessary in the common code.

At the same time move the remaining M2P define just above
set_gpfn_from_mfn to keep all the dummy helpers for M2P together.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>