mm, tile: drop arch_{add,remove}_memory

these functions are unreachable because tile doesn't support memory
hotplug becasuse it doesn't select ARCH_ENABLE_MEMORY_HOTPLUG nor
it supports SPARSEMEM.

This code hasn't been compiled for a while obviously because nobody has
noticed that __add_pages has a different signature since 2009.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>