utils: cleanup

Signed-off-by: Alban Gruin <alban.gruin@irit.fr>

src/utils.v

 Require Import Lia List Nat.
 Import ListNotations.
 
-Scheme Equality for list.
-
 Lemma list_elt_exists :
   forall (A : Type) (l : list A) (d : A) i,
     i < List.length l -> (exists a, List.nth_error l i = Some a).