Migration: fully delete the postmortem tool.

I removed the postmortem functionality in
203d0ed7897538ef600e09bf281306aa22e53338 but instead of removing the
postmortem tool itself, I merely removed the body of its `main`
method.  This CL finishes the removal.

Change-Id: I525afe40ab3b69b81edc28b0fdded06fed5710c6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204320
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
diff --git a/pkg/nnbd_migration/tool/postmortem.dart b/pkg/nnbd_migration/tool/postmortem.dart
deleted file mode 100644
index a523b3a..0000000
--- a/pkg/nnbd_migration/tool/postmortem.dart
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright (c) 2019, 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:args/args.dart';
-import 'package:meta/meta.dart';
-
-void main(List<String> args) {}
-
-class Subcommand {
-  final String name;
-  final String suffix;
-  final String help;
-  final ArgParser argParser;
-
-  Subcommand(
-      {@required this.name,
-      this.suffix,
-      @required this.help,
-      @required this.argParser});
-}