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

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

.

The prompt was removed in commit 2ec7e705 ("net: routing: Hide routing
option temporarily").

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