bluetooth: kconfig: Do not assign promptless BT_DEBUG

BT_DEBUG is a promptless helper symbol, defined in
subsys/bluetooth/common/Kconfig. It is selected by BT_DEBUG_LOG, which
these configuration files also enable.

Flagged by https://github.com/zephyrproject-rtos/zephyr/pull/20742

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