simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumconnect/queries/get_holidays/get_holidays.dart

@@ -4,10 +4,5 @@ import '../../models/mc_holiday.dart';
 class GetHolidays extends MarianumConnectQuery {
   GetHolidays({super.dio});
 
-  Future<List<McHoliday>> run() => guard(() async {
-    final response = await dio.get<List<dynamic>>(endpoint('holidays'));
-    return response.data!
-        .map((e) => McHoliday.fromJson(e as Map<String, dynamic>))
-        .toList();
-  });
+  Future<List<McHoliday>> run() => getList('holidays', McHoliday.fromJson);
 }