bonding: remove useless assignment

The variable old_active is first set to bond->curr_active_slave.
Then, it is unconditionally set to new_active, without being used in between.

The first assignment, having no side effect, is useless.

Signed-off-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Reviewed-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>