[Hotfix] Remove code in PushAgentBasic that presumes that there is only one brain

unity-environment/Assets/ML-Agents/Examples/PushBlock/Scripts/PushAgentBasic.cs

 
     void Awake()
     {
-        // There is one brain in the scene so this should find our brain.
-        brain = FindObjectOfType<Brain>();
         academy = FindObjectOfType<PushBlockAcademy>(); //cache the academy
     }