Bluetooth: audio: capabilities: Remove callbacks

This removes the callbacks from capabilities. The callbacks are used for
unicast server role only, while the capabilities are used for the
broadcast sink role as well. Thus the callbacks can be removed as there
is another bt_audio_unicast_server_cb API that is specific for unicast
server role.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>