Remove flag react_native_new_architecture:runtimescheduler_enable_yielding_ios

Summary:
changelog: [internal]

This is shipped on iOS, let's remove gating.

Reviewed By: javache

Differential Revision: D34144360

fbshipit-source-id: 8ca1b3a061470155405bf1b71cdc9ddda6fc2be3

Author: sammy-SC

React/Fabric/RCTSurfacePresenter.mm

@@ -279,8 +279,7 @@ - (RCTScheduler *)_createScheduler
   auto weakRuntimeScheduler = _contextContainer->find<std::weak_ptr<RuntimeScheduler>>("RuntimeScheduler");
   auto runtimeScheduler = weakRuntimeScheduler.hasValue() ? weakRuntimeScheduler.value().lock() : nullptr;
   if (runtimeScheduler) {
-    runtimeScheduler->setEnableYielding(
-        reactNativeConfig->getBool("react_native_new_architecture:runtimescheduler_enable_yielding_ios"));
+    runtimeScheduler->setEnableYielding(true);
     runtimeExecutor = [runtimeScheduler](std::function<void(jsi::Runtime & runtime)> &&callback) {
       runtimeScheduler->scheduleWork(std::move(callback));
     };