Fix linter warnings

Author: solid-maksymtielnyi

lib/src/expanded/subscription_expanded.dart

@@ -1,6 +1,5 @@
 import 'package:stripe/messages.dart';
 import 'package:stripe/src/expanded.dart';
-import 'package:stripe/src/messages/enums/expandable_fields/invoice_expandable_field.dart';
 import 'package:stripe/src/utils/expandable_fields/discounts_expandable_field.dart';
 import 'package:stripe/src/utils/expandable_fields/latest_invoice_expanded_expandable_field.dart';
 

lib/src/resources/subscription.dart

@@ -2,7 +2,6 @@ import 'dart:async';
 
 import 'package:stripe/messages.dart';
 import 'package:stripe/src/expanded.dart';
-import 'package:stripe/src/messages/enums/expandable_fields/invoice_expandable_field.dart';
 import 'package:stripe/src/utils/expandable_field.dart';
 import 'package:stripe/src/utils/expandable_fields/discounts_expandable_field.dart';
 import 'package:stripe/src/utils/expandable_fields/latest_invoice_expanded_expandable_field.dart';