Revert "better printing of returns"

It introduces a syntax error on the : in the produced assembly.js at
line 1636:
  var res_ref = function (r) {
    return
      {
>>      res_type: Coq_restype_normal(),
        res_value: Coq_resvalue_ref(r),
        res_label: Coq_label_empty()
      };
  };

This reverts commit 97c8a730b5f246ee3b3f5d8a5f510d16f18412d1.