Cleanup obsolete --strong option of front-end server (#13735)

Author: alexmarkov

testing/run_tests.py

@@ -148,7 +148,6 @@ def SnapshotTest(build_dir, dart_file, kernel_file_output, verbose_dart_snapshot
     '--sdk-root',
     flutter_patched_sdk,
     '--incremental',
-    '--strong',
     '--target=flutter',
     '--packages',
     test_packages,

testing/scenario_app/compile_ios_jit.sh

@@ -39,7 +39,7 @@ echo "Compiling to kernel..."
 "$HOST_TOOLS/dart" \
   "$HOST_TOOLS/gen/frontend_server.dart.snapshot" \
   --sdk-root "$HOST_TOOLS/flutter_patched_sdk" \
-  --strong --target=flutter \
+  --target=flutter \
   --no-link-platform \
   --output-dill "$OUTDIR/App.framework/flutter_assets/kernel_blob.bin" \
   "${BASH_SOURCE%/*}/lib/main.dart"