tests: net: kconfig: Do not assign promptless NET_ROUTE symbol

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

.

(There doesn't seem to be any way to enable this symbol, because it's
not being select'ed either.)

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