mm: remove obsolete alloc_remap()

Tile was the only remaining architecture to implement alloc_remap(),
and since that is being removed, there is no point in keeping this
function.

Removing all callers simplifies the mem_map handling.

Reviewed-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>