Don't try to handle regions in Flambda.fold_lets_option

It used to be much more necessary, but no more, and it interferes with the
handling of regions using `enter_region`.

middle_end/flambda/flambda.ml

@@ -911,10 +911,6 @@ module With_free_variables = struct
         free_vars_of_body;
       }
 
-  let create_region (t : expr t) =
-    let Expr (body, fv) = t in
-    Expr (Region body, fv)
-
   let expr (t : expr t) =
     match t with
     | Expr (expr, free_vars) -> Named (Expr expr, free_vars)
@@ -939,7 +935,7 @@ let fold_lets_option
   let finish ~last_body ~acc ~rev_lets =
     let module W = With_free_variables in
     let acc, t =
-      List.fold_left (fun (acc, t) (has_region, var, defining_expr) ->
+      List.fold_left (fun (acc, t) (var, defining_expr) ->
           let free_vars_of_body = W.free_variables t in
           let acc, var, defining_expr =
             filter_defining_expr acc var defining_expr free_vars_of_body
@@ -950,7 +946,6 @@ let fold_lets_option
             | Some defining_expr ->
               W.of_expr (W.create_let_reusing_body var defining_expr t)
           in
-          let t = if has_region then W.create_region t else t in
           acc, t)
         (acc, W.of_expr last_body)
         rev_lets
@@ -959,13 +954,11 @@ let fold_lets_option
   in
   let rec loop (t : t) ~acc ~rev_lets =
     match t with
-    | Let { var; defining_expr; body; _ }
-    | Region (Let { var; defining_expr; body; _ }) ->
+    | Let { var; defining_expr; body; _ } ->
       let acc, var, defining_expr =
         for_defining_expr acc var defining_expr
       in
-      let has_region = match t with Region _ -> true | _ -> false in
-      let rev_lets = (has_region, var, defining_expr) :: rev_lets in
+      let rev_lets = (var, defining_expr) :: rev_lets in
       loop body ~acc ~rev_lets
     | t ->
       let last_body, acc = for_last_body acc t in