configs: remove CONFIG_NET_MULTI

CONFIG_NET_MULTI is not used anywhere and thus can safely be removed
from the configs.

Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>