remove unnused variable

src/GraphVisJs/Graph.js

@@ -20,7 +20,7 @@ export class Graph extends React.Component {
     };
     var options = parseData.options;
     options.nodes = { font: { color : "white"}};
-    var network = new vis.Network(this.ref.current, data, options);
+    new vis.Network(this.ref.current, data, options);
   }
 
   componentDidMount() {