unregister on disabled

com.unity.ml-agents/Runtime/Agent.cs

                 NotifyAgentDone(DoneReason.Disabled);
             }
             m_Brain?.Dispose();
-            m_Initialized = false;
-        }
-
-        void OnDestroy()
-        {
+            m_Initialized = false;
         }
 
         void NotifyAgentDone(DoneReason doneReason)