configs: legoev3: disable CONFIG_NET

configs: legoev3: disable CONFIG_NET

This disables the CONFIG_NET setting for LEGO MINDSTORMS EV3. This board
does not have any built-in networking, so it does not make sense to
enable this feature.

This also fixes the warning:

===================== WARNING ======================
This board does not use CONFIG_DM_ETH (Driver Model
for Ethernet drivers). Please update the board to use
CONFIG_DM_ETH before the v2020.07 release. Failure to
update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

Signed-off-by: David Lechner <david@lechnology.com>