Sync scheduling by default, with an async opt-in (#11771)

Removes the `useSyncScheduling` option from the HostConfig, since it's
no longer needed. Instead of globally flipping between sync and async,
our strategy will be to opt-in specific trees and subtrees.

Author: acdlite

src/ReactTestRenderer.js

@@ -203,8 +203,6 @@ const TestRenderer = ReactFiberReconciler({
     clearTimeout(timeoutID);
   },
 
-  useSyncScheduling: true,
-
   getPublicInstance,
 
   now(): number {