Bluetooth: Audio: Add missing unbind of audio_iso for broadcast sink

When a BIS disconnected, we removed the references between
the endpoint and the stream. This made it impossible
to later unbind the audio_iso when
broadcast_sink_cleanup_streams was called.

Fixed by unbind the audio_iso when we remove the reference.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>