opt: sealed LoadingState (#765)

lib/pages/member_search/search_archive.dart

@@ -92,7 +92,6 @@ class _SearchArchiveState extends State<SearchArchive>
             widget.ctr.refreshArchive();
           },
         ),
-      LoadingState() => throw UnimplementedError(),
     };
   }
 

lib/pages/member_search/search_dynamic.dart

@@ -149,7 +149,6 @@ class _SearchDynamicState extends State<SearchDynamic>
             widget.ctr.refreshDynamic();
           },
         ),
-      LoadingState() => throw UnimplementedError(),
     };
   }