Remove out of date references to stale documentation

Excessive ref documentation like this goes out of date (the
documentation link is broken, and the JupyterHub link is also out of date
as newer versions comes out). I don't think we should add those inside
code.

oauthenticator/oauth2.py

@@ -1051,9 +1051,6 @@ async def authenticate(self, handler, data=None, **kwargs):
           (optional), and auth_state (optional)
 
         Subclasses should not override this method.
-
-        ref: https://jupyterhub.readthedocs.io/en/stable/reference/authenticators.html#authenticator-authenticate-method
-        ref: https://github.com/jupyterhub/jupyterhub/blob/4.0.0/jupyterhub/auth.py#L581-L611
         """
         # build the parameters to be used in the request exchanging the oauth code for the access token
         access_token_params = self.build_access_tokens_request_params(handler, data)