refactor(edit_headline_event): remove EditHeadlineExcerptChanged event

    This commit removes the `EditHeadlineExcerptChanged` event as the `excerpt`
    field is no longer part of the `Headline` model in the core package.

Author: fulleni

lib/content_management/bloc/edit_headline/edit_headline_event.dart

@@ -21,14 +21,6 @@ final class EditHeadlineTitleChanged extends EditHeadlineEvent {
   List<Object?> get props => [title];
 }
 
-/// Event for when the headline's excerpt is changed.
-final class EditHeadlineExcerptChanged extends EditHeadlineEvent {
-  const EditHeadlineExcerptChanged(this.excerpt);
-  final String excerpt;
-  @override
-  List<Object?> get props => [excerpt];
-}
-
 /// Event for when the headline's URL is changed.
 final class EditHeadlineUrlChanged extends EditHeadlineEvent {
   const EditHeadlineUrlChanged(this.url);