repair with traitlets 5.10

Index: notebook/services/sessions/sessionmanager.py
--- notebook/services/sessions/sessionmanager.py.orig
+++ notebook/services/sessions/sessionmanager.py
@@ -31,7 +31,6 @@ class SessionManager(LoggingConfigurable):
             # and allow jupyter_server contents managers to pass
             # through. If jupyter_server is not installed, this class
             # will be ignored.
-            'jupyter_server.services.contents.manager.ContentsManager'
         ]
     )