[Scheduler] Delete old rAF implementation (#17252)

We've been using the message loop version for a while. Time to delete.

Author: acdlite

packages/scheduler/src/SchedulerFeatureFlags.js

@@ -8,5 +8,4 @@
 
 export const enableSchedulerDebugging = false;
 export const enableIsInputPending = false;
-export const enableMessageLoopImplementation = true;
 export const enableProfiling = __PROFILE__;