Revert "Flag use-js-string default to false for wasm_of_ocaml"

This reverts commit a0133d0.

Author: vouillon

compiler/lib/config.ml

@@ -193,9 +193,7 @@ let target () =
 let set_target (t : [ `JavaScript | `Wasm ]) =
   (match t with
   | `JavaScript -> Targetint.set_num_bits 32
-  | `Wasm ->
-      Targetint.set_num_bits 31;
-      Flag.disable "use-js-string");
+  | `Wasm -> Targetint.set_num_bits 31);
   target_ := (t :> [ `JavaScript | `Wasm | `None ])
 
 type effects_backend =