configs: stm32f429-discovery: Disable CONFIG_NET flag

configs: stm32f429-discovery: Disable CONFIG_NET flag

Network support was never added on this board, disable
CONFIG_NET flag to avoid following 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: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Patrick Delaunay <patrick.delaunay@st.com>