Don't call, they might possibly try to charge you extra for cancelling, or because a one way is more expensive.
Keep watching the return leg. If the airline changes it's time several hours, or if they cancel the flight, you can request money back from the airline.
If that doesn't happen, just don't check in for the flight. They'll have plenty of others to put in your empty seat.
This all assumes it is a non-refundable fare, of course.