memblock: kill "config MAX_ACTIVE_REGIONS"

The Kconfig symbol MAX_ACTIVE_REGIONS is unused. Commit
0ee332c1

 ("memblock: Kill
early_node_map[]") removed the only place were it was actually used. But
it did not remove its Kconfig entries (for powerpc and sh).

Remove those two entries (and the entry for metag, that popped up in
v3.9-rc1).

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>