There never was one in the 2-vs-1 applet. It starts calculating when you press 'Apply' or 'Design ready'. For the 3-vs-1 applet a separate button was needed because you have to enter the moves of two pieces, and you don't want calculation to start after defining the moves of just one of the pieces. (As it takes quite long, and cannot be interrupted.)
There never was one in the 2-vs-1 applet. It starts calculating when you press 'Apply' or 'Design ready'. For the 3-vs-1 applet a separate button was needed because you have to enter the moves of two pieces, and you don't want calculation to start after defining the moves of just one of the pieces. (As it takes quite long, and cannot be interrupted.)