tests: net: all: kconfig: Do not assign promptless NET_RAW_MODE

Assignments have no effect on promptless symbols. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

This symbol is selected by IEEE802154_RAW_MODE, defined in
drivers/ieee802154/Kconfig.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>