repair with traitlets 5.10

Index: notebook/notebookapp.py
--- notebook/notebookapp.py.orig
+++ notebook/notebookapp.py
@@ -1388,7 +1388,6 @@ class NotebookApp(JupyterApp):
             # and allow jupyter_server contents managers to pass
             # through. If jupyter_server is not installed, this class
             # will be ignored.
-            'jupyter_server.contents.services.managers.ContentsManager'
         ],
         config=True,
         help=_('The notebook manager class to use.')