tests: net: kconfig: Do not assign promptless NET_L2_IEEE802154_MGMT
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 <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr