irmin-pack.unix: Remove calls to Index.Stats.add_* from Io.

This seems to be remnant from before `index` was split into its own
repository.

src/irmin-pack/unix/io.ml

@@ -168,7 +168,6 @@ module Unix = struct
            usage is safe. *)
         let buf = Bytes.unsafe_of_string s in
         let () = Util.really_write t.fd off buf 0 len in
-        Index.Stats.add_write len;
         ()
 
   let write_string t ~off s =
@@ -195,7 +194,6 @@ module Unix = struct
     | true -> raise Errors.Closed
     | false ->
         let nread = Util.really_read t.fd off len buf in
-        Index.Stats.add_read nread;
         if nread <> len then
           (* didn't manage to read the desired amount; in this case the interface seems to
              require we return `Read_out_of_bounds FIXME check this, because it is unusual
@@ -232,7 +230,6 @@ module Unix = struct
           ~length:len
       in
       if nread > 0 then (
-        Index.Stats.add_read nread;
         Buffer.add_subbytes buf bytes 0 nread;
         if nread = len then aux ~off:Int63.(add off (of_int nread)) count
         else count)