Use Yojson.Safe.t list as placeholder type for storage_transformers.

Since the storage_transformer metadata is not yet supported, we use
a trivial `Yojson.Safe.t list` instead of a phantom specialized type.

zarr/src/metadata.ml

@@ -111,7 +111,7 @@ module ArrayMetadata = struct
     ;chunk_key_encoding : ChunkKeyEncoding.t
     ;attributes : Yojson.Safe.t
     ;dimension_names : string option list
-    ;storage_transformers : Yojson.Safe.t Util.ExtPoint.t list}
+    ;storage_transformers : Yojson.Safe.t list}
 
   let create
     ?(sep=`Slash)

zarr/src/util.ml

@@ -1,13 +1,3 @@
-module ExtPoint = struct
-  type 'a t =
-    {name : string
-    ;configuration : 'a}
-
-  let ( = ) cmp x y =
-    (x.name = y.name) &&
-    cmp x.configuration y.configuration
-end
-
 module ArrayMap = struct
   include Map.Make (struct
     type t = int array

zarr/src/util.mli

@@ -1,10 +1,3 @@
-module ExtPoint : sig
-  (** The type representing a JSON extension point metadata configuration. *)
-
-  type 'a t = {name : string ; configuration : 'a}
-  val ( = ) : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool
-end
-
 (** A finite map over integer array keys. *)
 module ArrayMap : sig
   include Map.S with type key = int array