tests: net: kconfig: Do not assign promptless NET_L2_IEEE802154_MGMT

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

.

This symbol is already enabled indirectly through being selected by
NET_L2_IEEE802154_RFD, which is enabled in the configuration file.

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