Change guide to not extend two bases classes in c# (#2104)

Documentation change to Heuristic Brain

docs/Learning-Environment-Design-Heuristic-Brains.md

 
 ## Implementing the Decision interface
 
-When creating your Decision class, extend MonoBehaviour (so you can use the
-class as a Unity component) and extend the Decision interface.
-
 ```csharp
 using UnityEngine;
 using MLAgents;