Bluetooth: audio - remove BT_AUDIO_DEBUG

Remove the BT_AUDIO_DEBUG Kconfig option.

Given that all the audio modules are guarded by the BT_AUDIO config,
it seems excessive to also have a separate guard for audio module
debug. It is sufficient that this is turned on/off module by module.

Note that the BT_AUDIO_DEBUG was also located in the wrong file - the
babs Kconfig file, not the main Kconfig file.

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>