Remove unused import

lib/src/minio_sign.dart

@@ -2,7 +2,6 @@ import 'package:convert/convert.dart';
 import 'package:crypto/crypto.dart';
 import 'package:minio/minio.dart';
 import 'package:minio/src/minio_client.dart';
-import 'package:minio/src/minio_errors.dart';
 import 'package:minio/src/minio_helpers.dart';
 import 'package:minio/src/utils.dart';