There is no way to do that now, but the reason behind this is basically because the variables shown there are actually shared between all graphs of the agent/gameobject as well as Sub-Graphs. Thus the only way that this could be implemented would be to show if the variables are used by the current graph viewed in the editor only, but it could still be used by some other graph of the gameobject or sub-graph. This though can potentially gets confusing and create accidentally deleted variables.
Thus, I will have to think about this and if there is a clean way, UI wise, to implement it 🙂