refactor(content_management): remove unused country loading event

- Remove EditHeadlineLoadMoreCountriesRequested event from edit_headline_event.dart
- This event was likely intended for loading more countries but is no longer needed

Author: fulleni

lib/content_management/bloc/edit_headline/edit_headline_event.dart

@@ -83,8 +83,3 @@ final class EditHeadlineStatusChanged extends EditHeadlineEvent {
 final class EditHeadlineSubmitted extends EditHeadlineEvent {
   const EditHeadlineSubmitted();
 }
-
-/// Event to request loading more countries.
-final class EditHeadlineLoadMoreCountriesRequested extends EditHeadlineEvent {
-  const EditHeadlineLoadMoreCountriesRequested();
-}