Why in helicopter autorotation phase the opposing torque is eliminated?
Because the engine is not applying power to the rotors. No engine power means there will be no torque pushing the airframe in the opposite direction.

In auto, the main rotor essentially becomes a pinwheel, kept moving by the air rushing past it... 

Auto doesn't really power the rotors, it keeps them moving so that the spinning rotor creates air resistance and slows the descent of the helicopter, and maintains enough inertia in the blades to flare and stop the descent. 

Of course, that inertia is only sufficient for one attempt at flaring and stopping descent.