delete obsolete scanner roundtrip test

pkg/front_end/test/scanner_roundtrip_test.dart

-// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-import 'package:analyzer/src/fasta/token_utils.dart';
-import 'package:front_end/src/scanner/errors.dart' as analyzer;
-import 'package:front_end/src/scanner/reader.dart' as analyzer;
-import 'package:front_end/src/scanner/scanner.dart' as analyzer;
-import 'package:front_end/src/scanner/token.dart';
-import 'package:test/test.dart';
-import 'package:test_reflective_loader/test_reflective_loader.dart';
-
-import 'scanner_fasta_test.dart';
-import 'scanner_test.dart';
-
-main() {
-  // round trip test removed
-}
-
-class TestScanner extends analyzer.Scanner {
-  TestScanner(analyzer.CharacterReader reader) : super.create(reader);
-
-  @override
-  void reportError(
-      analyzer.ScannerErrorCode errorCode, int offset, List<Object> arguments) {
-    fail('Unexpected error $errorCode while scanning offset $offset\n'
-        '   arguments: $arguments');
-  }
-}