drivers: eth: native: Remove automatic start script support

As the automatic start requires that the zephyr exe is run as
a root user, it is somewhat cumbersome to use. Remove the script
support and require that the zeth device is created beforehand
by the net-setup.sh script found in net-tools zephyr project.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>