Removal of useless lemma introduced in previous commit

Util/ListComplements.v

@@ -52,13 +52,6 @@ intros x l Hin. induction l.
 - destruct Hin. subst. now left. right. auto.
 Qed. *)
 
-Lemma hd_proof : forall l : list A, 0 < length l -> A.
-Proof using .
-  intros * H. destruct l as [|a t]. 2: exact a. exfalso.
-  rewrite (proj2 (length_zero_iff_nil _)) in H by reflexivity.
-  inversion H.
-Qed.
-
 Lemma all_eq : forall (l : list A) (a1 : A), In a1 l ->length l = 1
   -> forall a2 : A, In a2 l -> a2 = a1.
 Proof using .