configs: stm32f469-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>

configs/stm32f469-discovery_defconfig

@@ -18,12 +18,12 @@ CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SF=y
 # CONFIG_CMD_SETEXPR is not set
-# CONFIG_CMD_MII is not set
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_TIMER=y
 # CONFIG_ISO_PARTITION is not set
 CONFIG_OF_CONTROL=y
 CONFIG_DEFAULT_DEVICE_TREE="stm32f469-disco"
+# CONFIG_NET is not set
 CONFIG_DM_MMC=y
 CONFIG_ARM_PL180_MMCI=y
 CONFIG_MTD=y