Remove redefenition of username_claim in Generic

oauthenticator/generic.py

@@ -60,20 +60,6 @@ def _login_service_default(self):
         """,
     )
 
-    username_claim = Union(
-        [Unicode(os.environ.get('OAUTH2_USERNAME_KEY', 'username')), Callable()],
-        config=True,
-        help="""
-        When `userdata_url` returns a json response, the username will be taken
-        from this key.
-
-        Can be a string key name or a callable that accepts the returned
-        userdata json (as a dict) and returns the username.  The callable is
-        useful e.g. for extracting the username from a nested object in the
-        response.
-        """,
-    )
-
     @default("http_client")
     def _default_http_client(self):
         return AsyncHTTPClient(