Bluetooth: audio: tbs_client: Subscribe to all required notifications

This removes odd `subscribe` parameter from bt_tbs_client_discover
function parameters list. As the API user enables specific
CONFIG_BT_TBS_CLIENT_* option, the client implementation should
subscribe to the characteristic notifications so that the user receives
the value updates.

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