Bluetooth: Remove legacy debug symbols

The `BT_DEBUG_*` Kconfig symbols have been deprecated for more than 2
versions, remove them.

Update code that was still using them.

Remove the Bluetooth specific `Kconfig.template.log_config_bt` and use
`Kconfig.template.log_config_inherit` from the logging subsystem
instead, now that the legacy symbols can be removed.

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