unext: fix adaptation-set-switching mismatching

See: shaka-project#1402

packager/mpd/base/period.cc

@@ -145,12 +145,9 @@ std::optional<xml::XmlNode> Period::GetXml(bool output_period_duration) {
   }
 
   // Iterate thru AdaptationSets and add them to one big Period element.
-  // Also force AdaptationSets Id to incremental order, which might not
-  // be the case if force_cl_index is used.
-  int idx = 0;
   for (const auto& adaptation_set : adaptation_sets_) {
     auto child = adaptation_set->GetXml();
-    if (!child || !child->SetId(idx++) || !period.AddChild(std::move(*child)))
+    if (!child || !period.AddChild(std::move(*child)))
       return std::nullopt;
   }