Bluetooth: Logging: Make `BT_LOG` and `BT_LOG_LEGACY` hidden symbols

Make `BT_LOG` and `BT_LOG_LEGACY` hidden Kconfig symbols.

They should not be used by the user to configure the Bluetooth logging
system. If the user want to completely disable Bluetooth logging, they
should use `BT_LOG_LEVEL_OFF=y`.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>