Remove outdated React async component check

Summary: I added this check [a couple of years ago](1b22d49ae8) to mimic how [React used to check for async roots](acabf11245/packages/react-reconciler/src/ReactFiberReconciler.js (L321-L330)). This code doesn't make sense anymore since there's neither an async base class or an `unstable_ConcurrentMode` export, so I'm just cleaning it up.

Reviewed By: threepointone, sebmarkbage

Differential Revision: D16668567

fbshipit-source-id: 5ccf5feccc4b65ffb3aeb0a09891d8be7490df26

Libraries/ReactNative/renderApplication.js

@@ -33,7 +33,7 @@ function renderApplication<Props: Object>(
 ) {
   invariant(rootTag, 'Expect to have a valid rootTag, instead got ', rootTag);
 
-  let renderable = (
+  const renderable = (
     <PerformanceLoggerContext.Provider
       value={scopedPerformanceLogger ?? GlobalPerformanceLogger}>
       <AppContainer rootTag={rootTag} WrapperComponent={WrapperComponent}>
@@ -45,21 +45,6 @@ function renderApplication<Props: Object>(
     </PerformanceLoggerContext.Provider>
   );
 
-  // If the root component is async, the user probably wants the initial render
-  // to be async also. To do this, wrap AppContainer with an async marker.
-  // For more info see https://fb.me/is-component-async
-  if (
-    /* $FlowFixMe(>=0.68.0 site=react_native_fb) This comment suppresses an
-     * error found when Flow v0.68 was deployed. To see the error delete this
-     * comment and run Flow. */
-    RootComponent.prototype != null &&
-    RootComponent.prototype.unstable_isAsyncReactComponent === true
-  ) {
-    // $FlowFixMe This is not yet part of the official public API
-    const ConcurrentMode = React.unstable_ConcurrentMode;
-    renderable = <ConcurrentMode>{renderable}</ConcurrentMode>;
-  }
-
   GlobalPerformanceLogger.startTimespan('renderApplication_React_render');
   if (fabric) {
     require('../Renderer/shims/ReactFabric').render(renderable, rootTag);