Add and use BackgroundId::is_local.

td/telegram/BackgroundId.h

@@ -42,6 +42,10 @@ class BackgroundId {
     return id != 0;
   }
 
+  bool is_local() const {
+    return 0 < id && id <= 0x7FFFFFFF;
+  }
+
   template <class StorerT>
   void store(StorerT &storer) const {
     td::store(id, storer);

td/telegram/BackgroundManager.cpp

@@ -753,7 +753,7 @@ void BackgroundManager::remove_background(BackgroundId background_id, Promise<Un
       });
 
   if (!background->type.has_file()) {
-    if (background->is_default) {
+    if (!background->id.is_local()) {
       return td_->create_handler<UnsaveBackgroundQuery>(std::move(query_promise))
           ->send(telegram_api::make_object<telegram_api::inputWallPaperNoFile>(background_id.get()));
     } else {