refactor(content_management): remove unused country search event

- Remove EditHeadlineCountrySearchChanged event from edit_headline_event.dart
- This event was likely unused and unnecessary, simplifying the event structure

Author: fulleni

lib/content_management/bloc/edit_headline/edit_headline_event.dart

@@ -84,14 +84,6 @@ final class EditHeadlineSubmitted extends EditHeadlineEvent {
   const EditHeadlineSubmitted();
 }
 
-/// Event for when the country search term is changed.
-final class EditHeadlineCountrySearchChanged extends EditHeadlineEvent {
-  const EditHeadlineCountrySearchChanged(this.searchTerm);
-  final String searchTerm;
-  @override
-  List<Object?> get props => [searchTerm];
-}
-
 /// Event to request loading more countries.
 final class EditHeadlineLoadMoreCountriesRequested extends EditHeadlineEvent {
   const EditHeadlineLoadMoreCountriesRequested();