remove cookie_secret from single-user servers

pass encrypted cookies to Hub for verification

jupyterhub/orm.py

@@ -299,7 +299,6 @@ class User(Base):
             hub = db.query(Hub).first()
         self.server = Server(
             cookie_name='%s-%s' % (hub.server.cookie_name, self.name),
-            cookie_secret=hub.server.cookie_secret,
             base_url=url_path_join(base_url, 'user', self.name),
         )
         db.add(self.server)