remove preventDefault on menubar

no longer needed after removing fixed header

IPython/html/static/notebook/js/menubar.js

@@ -62,7 +62,6 b' define(['
                 // re-select it upon selection.
                 var i = that.notebook.get_selected_index();
                 that.notebook.select(i);
-                event.preventDefault();
             }
         );
     };