Remove flushSync from React Native (#12565)

There are no plans to enable async in the old renderer. In the new renderer
it only really makes sense to do from the main thread and probably from
native since it'll have to yield to native first.

Author: sebmarkbage

packages/react-native-renderer/src/ReactFabric.js

@@ -77,8 +77,6 @@ const ReactFabric: ReactNativeType = {
 
   unstable_batchedUpdates: ReactGenericBatching.batchedUpdates,
 
-  flushSync: ReactFabricRenderer.flushSync,
-
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: {
     // Used as a mixin in many createClass-based components
     NativeMethodsMixin,

packages/react-native-renderer/src/ReactNativeRenderer.js

@@ -96,8 +96,6 @@ const ReactNativeRenderer: ReactNativeType = {
 
   unstable_batchedUpdates: ReactGenericBatching.batchedUpdates,
 
-  flushSync: ReactNativeFiberRenderer.flushSync,
-
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: {
     // Used as a mixin in many createClass-based components
     NativeMethodsMixin,