refactor(headlines-feed): simplify state management in TopicsFilterBloc

- Remove redundant error field when clearing followed topics error
- Replace specific error with a more generic clearError flag
- Update state copy operation to use the new clearError flag

Author: fulleni

lib/headlines-feed/bloc/topics_filter_bloc.dart

@@ -151,8 +151,7 @@ class TopicsFilterBloc extends Bloc<TopicsFilterEvent, TopicsFilterState> {
           state.copyWith(
             followedTopicsStatus: TopicsFilterStatus.success,
             followedTopics: const [],
-            error: const OperationFailedException('No followed topics found.'),
-            clearFollowedTopicsError: true,
+            clearError: true,
           ),
         );
         return;