There's a trickle-down effect when an NHL team is eliminated from the playoffs. Calgary was swept by Anaheim, which means they didn't need their extra goalie anymore.
Jon Gillies was sent back to Stockton and the Heat returned Mason McDonald to the Adirondack. The Thunder have two established goalies again.
The Thunder had been using Connor Hicks, who just finished his first year of juniors, as a backup while McDonald was gone. (Due to roster restrictions they could not add a goalie who had already played for the team)
J.P. Anderson has been playing very well ever since he returned to the Thunder in March, so it seems likely he'll keep playing. Anderson's goals-against average of 2.03 is fifth in the playoffs. McDonald does provide a strong backup plan in case of injury.