Bluetooth: Audio: Remove PAC_TYPE_UNUSED

The PAC_TYPE_UNUSED would be breaking
enum rules, as it is setting and comparing
a value outside the enum range.

Instead we check if the `dir` field has been
set to SINK or SOURCE.

The reason why this still works, is that
we memset the struct unicast_client_pac
causing `dir` to become 0. This still
does not really follow the rules of enums, but
it is the best we can do without adding another
value to determine if a struct unicast_client_pac
is unused or not, without adding another value to
public enum struct.

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