fix(learnocaml_common.ml): Remove unused var (leftover of PR #489)

This small patch avoids a warning and also saves one HTTP request.

Author: erikmd

src/app/learnocaml_common.ml

@@ -1137,8 +1137,6 @@ let get_token ?(has_server = true) () =
         Lwt.return
     with
     Not_found ->
-      retrieve (Learnocaml_api.Nonce ())
-      >>= fun nonce ->
       ask_string ~title:"Token"
         [H.txt [%i"Enter your token"]]
       >>= fun input_tok ->