Remove unnecessary inheritance from GraphElementPresenter in NodePreviewPresenter

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Presenters/NodePreviewPresenter.cs

 namespace UnityEditor.MaterialGraph.Drawing
 {
     [Serializable]
-    public class NodePreviewPresenter : GraphElementPresenter
+    public class NodePreviewPresenter : ScriptableObject
     {
         protected NodePreviewPresenter()
         {}