Disable integration with UnityEditor.Undo system.

I couldn't get UnityEditor.Undo.ClearUndo() to work, which means that
the Undo/Redo commands in the Unity Editor Edit menu aren't cleared out
when the object they're supposed to undo/redo is destroyed. I don't want
phantom Undo/Redo commands hanging around after the custom editor window
is closed so I've disabled integration of CommandHistory with
UnityEditor.Undo for the time being.