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

Corresponding cleanup in flutter tools:
https://github.com/flutter/flutter/commit/cf6a8d6a4727919436daed8444a273ea7ffe8d73

Cleanup in the flutter engine:
https://github.com/flutter/engine/commit/ddceed5f7af13531ec38d522916736c0e42e4619

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>
diff --git a/pkg/frontend_server/lib/frontend_server.dart b/pkg/frontend_server/lib/frontend_server.dart
index 10eab7b..176909e 100644
--- a/pkg/frontend_server/lib/frontend_server.dart
+++ b/pkg/frontend_server/lib/frontend_server.dart
@@ -53,8 +53,6 @@
   ..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.',