refactor: Removed unused Button widget

Author: mateusz-bak

lib/ui/books_screen/widgets/add_book_method_button.dart

@@ -1,7 +1,6 @@
 export 'book_card_list.dart';
 export 'book_card_list_compact.dart';
 export 'books_list.dart';
-export 'add_book_method_button.dart';
 export 'year_filter_chip.dart';
 export 'tag_filter_chip.dart';
 export 'books_grid.dart';

lib/ui/books_screen/widgets/widgets.dart

@@ -23,7 +23,6 @@ class SetChallengeBox extends StatelessWidget {
     return Card(
       shadowColor: Colors.transparent,
       shape: RoundedRectangleBorder(
-        side: BorderSide(color: dividerColor, width: 1),
         borderRadius: BorderRadius.circular(cornerRadius),
       ),
       color: Theme.of(context).colorScheme.surfaceVariant,