UI: Delete backup file if deleting scene collection

I forgot to make sure that the backup file for a scene collection would
also be deleted if a scene collection is purposely deleted by the user.

obs/window-basic-main-scene-collections.cpp

@@ -310,6 +310,8 @@ void OBSBasic::on_actionRemoveSceneCollection_triggered()
 	oldFile.insert(0, path);
 	oldFile += ".json";
 	os_unlink(oldFile.c_str());
+	oldFile += ".bak";
+	os_unlink(oldFile.c_str());
 
 	Load(newPath.c_str());
 	RefreshSceneCollections();