Don't nf_univ_variables in prepare_hint

Seems not useful and may result in a speedup

cf https://coq.zulipchat.com/#narrow/stream/237656-Coq-devs-.26-plugin-devs/topic/Universe.20normalization.20in.20.60make_local_hint_db.60

tactics/hints.ml

@@ -1411,7 +1411,6 @@ let prepare_hint env init (sigma,c) =
   (* We re-abstract over uninstantiated evars and universes.
      It is actually a bit stupid to generalize over evars since the first
      thing make_resolves will do is to re-instantiate the products *)
-  let sigma = Evd.nf_univ_variables sigma in
   let c = Evarutil.nf_evar sigma c in
   let c = drop_extra_implicit_args sigma c in
   let vars = ref (collect_vars sigma c) in