Breaking: Drop the signature Backoff

One can instead write `module type of Backoff` to get a signature for the
`Backoff` module.

Author: polytypic

src/kcas.ml

@@ -4,8 +4,6 @@ Copyright (c) 2017, Nicolas ASSOUAD <nicolas.assouad@ens.fr>
 ########
 *)
 
-module type Backoff = Backoff.S
-
 module Backoff = Backoff.M
 
 module Id = struct

src/kcas.mli

@@ -59,9 +59,8 @@ val incr : int ref -> unit
 val decr : int ref -> unit
 (** [decr r] atomically decrements [r] *)
 
-(** {2 Backoff}
-    Suspend domains with exponential backoff. *)
-module type Backoff = sig
+(** Suspend domains with exponential backoff. *)
+module Backoff : sig
   type t
   (** The type of backoff value *)
 
@@ -79,5 +78,3 @@ module type Backoff = sig
   val reset : t -> unit
   (** Resets the backoff clock to 1 millisecond. *)
 end
-
-module Backoff : Backoff