deregister the same method we registered (#3072)

UnitySDK/Assets/ML-Agents/Scripts/Agent.cs

                 academy.AgentSendState -= SendInfo;
                 academy.DecideAction -= DecideAction;
                 academy.AgentAct -= AgentStep;
-                academy.AgentForceReset -= ForceReset;
+                academy.AgentForceReset -= _AgentReset;
             }
             m_Brain?.Dispose();
         }