Update type_checker.dart

Remove unnecessary comment

Author: fshcheglov

source_gen/lib/src/type_checker.dart

@@ -2,8 +2,6 @@
 // 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.
 
-// ignore_for_file: deprecated_member_use until analyzer 7 support is dropped.
-
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/constant/value.dart';