Fix a compile warning in GraphEditorView

com.unity.shadergraph/Editor/Drawing/Views/GraphEditorView.cs

         MaterialGraphView m_GraphView;
         MasterPreviewView m_MasterPreviewView;
 
-        private EditorWindow m_EditorWindow;
-
         AbstractMaterialGraph m_Graph;
         PreviewManager m_PreviewManager;
         SearchWindowProvider m_SearchWindowProvider;
         {
             m_Graph = graph;
             AddStyleSheetPath("Styles/GraphEditorView");
-            m_EditorWindow = editorWindow;
             previewManager = new PreviewManager(graph);
 
             string serializedWindowLayout = EditorUserSettings.GetConfigValue(k_FloatingWindowsLayoutKey);