powerpc/cell: Drop cbe-oss-dev mailing list from MAINTAINERS

Traffic on the cbe-oss-dev list is more or less non-existent, other than
CC's from linuxppc.

It's seems like we may as well just send everyone to linuxppc and
archive the list.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Jeremy Kerr <jk@ozlabs.org>