Revert "Merge branch 'byte-countable' into 'master'"

This reverts merge request !155 (merged) because it's incompatible with Coq ≤8.9.

theories/strings.v

 From Coq Require Import Ascii.
-From Coq Require Import Init.Byte.
 From Coq Require Export String.
 From stdpp Require Export list.
 From stdpp Require Import countable.
@@ -19,7 +18,6 @@ Arguments String.append : simpl never.
 
 (** * Decision of equality *)
 Instance ascii_eq_dec : EqDecision ascii := ascii_dec.
-Instance byte_eq_dec : EqDecision byte := Byte.byte_eq_dec.
 Instance string_eq_dec : EqDecision string.
 Proof. solve_decision. Defined.
 Instance string_app_inj : Inj (=) (=) (String.append s1).
@@ -121,5 +119,3 @@ Lemma ascii_of_to_digits a : ascii_of_digits (ascii_to_digits a) = a.
 Proof. by destruct a as [[][][][][][][][]]. Qed.
 Instance ascii_countable : Countable ascii :=
   inj_countable' ascii_to_digits ascii_of_digits ascii_of_to_digits.
-Instance byte_countable : Countable byte :=
-  inj_countable Byte.to_N Byte.of_N Byte.of_to_N.