Remove buggy test to check if tasks can be posted on all native threads...

Remove buggy test to check if tasks can be posted on all native threads recursively from the emebdder. (#16214)

This was never thread safe to shutdown of the concurrent message loop.

shell/platform/embedder/tests/embedder_unittests.cc

@@ -4006,40 +4006,5 @@ TEST_F(EmbedderTest, CanPostTaskToAllNativeThreads) {
   ASSERT_FALSE(engine.is_valid());
 }
 
-TEST_F(EmbedderTest, CanPostTaskToAllNativeThreadsRecursively) {
-  EmbedderConfigBuilder builder(GetEmbedderContext());
-
-  builder.SetSoftwareRendererConfig();
-
-  static std::mutex engine_mutex;
-  static UniqueEngine engine;
-  static fml::AutoResetWaitableEvent event;
-
-  std::unique_lock engine_lock(engine_mutex);
-  engine.reset();
-  engine = builder.LaunchEngine();
-  ASSERT_TRUE(engine.is_valid());
-  ASSERT_EQ(FlutterEnginePostCallbackOnAllNativeThreads(
-                engine.get(),
-                [](FlutterNativeThreadType type, void* baton) {
-                  // This should deadlock if the task mutex acquisition is
-                  // busted.
-                  std::scoped_lock engine_lock_inner(engine_mutex);
-                  if (engine.is_valid()) {
-                    ASSERT_EQ(FlutterEnginePostCallbackOnAllNativeThreads(
-                                  engine.get(),
-                                  [](FlutterNativeThreadType type,
-                                     void* baton) { event.Signal(); },
-                                  nullptr),
-                              kSuccess);
-                  }
-                },
-                &engine),
-            kSuccess);
-  engine_lock.unlock();
-  event.Wait();
-  engine.reset();
-}
-
 }  // namespace testing
 }  // namespace flutter