refactor(shared): remove unused LocalAdX extension

- Remove LocalAdX extension from local_ad_to_ad_type.dart
- Remove export of local_ad_to_ad_type.dart from extensions.dart
- Delete local_ad_to_ad_type.dart file

Author: fulleni

lib/shared/extensions/extensions.dart

@@ -6,6 +6,5 @@ export 'content_status_l10n.dart';
 export 'dashboard_user_role_l10n.dart';
 export 'feed_decorator_type_l10n.dart';
 export 'in_article_ad_slot_type_l10n.dart';
-export 'local_ad_to_ad_type.dart';
 export 'source_type_l10n.dart';
 export 'string_truncate.dart';