[front-end server] Remove obsolete --strong option

Corresponding cleanup in flutter tools:
flutter/flutter@cf6a8d6

Cleanup in the flutter engine:
flutter/engine@ddceed5

Change-Id: Idbc2d42aeb53e461e48522cb961feca30de8f1cc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124209
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

Author: alexmarkov

pkg/frontend_server/lib/frontend_server.dart

@@ -53,8 +53,6 @@ ArgParser argParser = ArgParser(allowTrailingOptions: true)
   ..addFlag('aot',
       help: 'Run compiler in AOT mode (enables whole-program transformations)',
       defaultsTo: false)
-// TODO(alexmarkov): Cleanup uses in Flutter and remove these obsolete flags.
-  ..addFlag('strong', help: 'Obsolete', defaultsTo: true)
   ..addFlag('tfa',
       help:
           'Enable global type flow analysis and related transformations in AOT mode.',