Delete the .notebook attribute for notebook panels again.

packages/notebook/src/panel.ts

@@ -137,16 +137,6 @@ class NotebookPanel extends DocumentWidget<Notebook, INotebookModel> {
    */
   readonly content: Notebook;
 
-  /**
-   * The notebook used by the widget.
-   *
-   * #### Notes
-   * This attribute is deprecated. Use `.content` instead.
-   */
-  get notebook(): Notebook {
-    return this.content;
-  }
-
   /**
    * The model for the widget.
    */