refactor(headlines-feed): remove unused TopicsFilterApplyFollowedRequested event

Removed the unused TopicsFilterApplyFollowedRequested event class from the topics filter bloc. This event was intended to apply the user's followed topics as filters but is no longer needed in the current implementation.

- Deleted TopicsFilterApplyFollowedRequested class
- Updated file to remove unnecessary code and improve maintainability

Author: fulleni

lib/headlines-feed/bloc/topics_filter_event.dart

@@ -20,8 +20,3 @@ final class TopicsFilterRequested extends TopicsFilterEvent {}
 /// Event triggered to request the next page of topics for pagination.
 /// {@endtemplate}
 final class TopicsFilterLoadMoreRequested extends TopicsFilterEvent {}
-
-/// {@template topics_filter_apply_followed_requested}
-/// Event triggered to request applying the user's followed topics as filters.
-/// {@endtemplate}
-final class TopicsFilterApplyFollowedRequested extends TopicsFilterEvent {}