tests: bluetooth: kconfig: Do not assign promptless BT_CONN symbol

Assignments have no effect on promptless symbols. This symbol is enabled
through being select'ed. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

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