tests: all: kconfig: Do not assign promptless NET_L2_IEEE802154_SUB_GHZ

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

.

This symbol is enabled through being selected by IEEE802154_CC1200 and
IEEE802154_RF2XX.

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