Cleanup unused unstable_startTransition, unstable_useTransition, unstable_useDeferredValue exports from fb packages (#27056)

## Summary

came across these TODOs – an internal grep indicated that remaining
callsites have been cleaned up, so these can now be removed.

## How did you test this change?

```
yarn flow dom-browser
yarn test
```

Author: noahlemen

fixtures/concurrent/time-slicing/src/index.js

@@ -1,4 +1,4 @@
-import React, {PureComponent, unstable_startTransition} from 'react';
+import React, {PureComponent, startTransition} from 'react';
 import {createRoot} from 'react-dom/client';
 import _ from 'lodash';
 import Charts from './Charts';
@@ -64,7 +64,7 @@ class App extends PureComponent {
     }
     this._ignoreClick = true;
 
-    unstable_startTransition(() => {
+    startTransition(() => {
       this.setState({showDemo: true}, () => {
         this._ignoreClick = false;
       });
@@ -103,7 +103,7 @@ class App extends PureComponent {
         break;
       case 'async':
         // TODO: useTransition hook instead.
-        unstable_startTransition(() => {
+        startTransition(() => {
           this.setState({value});
         });
         break;

packages/react/index.classic.fb.js

@@ -32,7 +32,6 @@ export {
   memo,
   cache,
   startTransition,
-  startTransition as unstable_startTransition, // TODO: Remove once call sights updated to startTransition
   unstable_Cache,
   unstable_TracingMarker,
   unstable_DebugTracingMode,
@@ -48,7 +47,6 @@ export {
   useContext,
   useDebugValue,
   useDeferredValue,
-  useDeferredValue as unstable_useDeferredValue, // TODO: Remove once call sights updated to useDeferredValue
   useEffect,
   experimental_useEffectEvent,
   useImperativeHandle,
@@ -61,7 +59,6 @@ export {
   useState,
   useSyncExternalStore,
   useTransition,
-  useTransition as unstable_useTransition, // TODO: Remove once call sights updated to useTransition
   version,
 } from './src/React';
 export {jsx, jsxs, jsxDEV} from './src/jsx/ReactJSX';

packages/react/index.modern.fb.js

@@ -31,7 +31,6 @@ export {
   memo,
   cache,
   startTransition,
-  startTransition as unstable_startTransition, // TODO: Remove once call sights updated to startTransition
   unstable_Cache,
   unstable_DebugTracingMode,
   unstable_LegacyHidden,
@@ -46,7 +45,6 @@ export {
   useContext,
   useDebugValue,
   useDeferredValue,
-  useDeferredValue as unstable_useDeferredValue, // TODO: Remove once call sights updated to useDeferredValue
   useEffect,
   experimental_useEffectEvent,
   useImperativeHandle,
@@ -59,7 +57,6 @@ export {
   useState,
   useSyncExternalStore,
   useTransition,
-  useTransition as unstable_useTransition, // TODO: Remove once call sights updated to useTransition
   version,
 } from './src/React';
 export {jsx, jsxs, jsxDEV} from './src/jsx/ReactJSX';