refactor(library/data/fintype/function): cleanup proof

library/data/fintype/function.lean

@@ -261,9 +261,8 @@ lemma list_eq_map_list_to_fun  (l : list B) (leq : length l = card A)
         assert Plt3 : find (kth k (elems A) Plt1) (elems A) < length l,
           {rewrite leq, apply find_kth},
         rewrite [kth_of_map Plt1 Plt2, list_to_fun_apply l leq _ Plt3],
-        generalize Plt3,
+        congruence,
         rewrite [find_kth_of_nodup Plt1 (unique A)],
-        intro Plt, exact rfl
       end
 
 lemma fun_to_list_to_fun (f : A → B) : ∀ P, list_to_fun (fun_to_list f) P = f :=