[UMD] Remove umd builds (#28735)

In React 19 React will finally stop publishing UMD builds. This is
motivated primarily by the lack of use of UMD format and the added
complexity of maintaining build infra for these releases. Additionally
with ESM becoming more prevalent in browsers and services like esm.sh
which can host React as an ESM module there are other options for doing
script tag based react loading.

This PR removes all the UMD build configs and forks.

There are some fixtures that still have references to UMD builds however
many of them already do not work (for instance they are using legacy
features like ReactDOM.render) and rather than block the removal on
these fixtures being brought up to date we'll just move forward and fix
or removes fixtures as necessary in the future.

DiffTrain build for [da6ba53](da6ba53)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-4ca20fd36b2444a74279e7022f89894710b1daab
+da6ba53b10d8240fc251ba14a3e5878604d3dc7d

compiled/facebook-www/ReactDOMTesting-dev.classic.js

@@ -36869,7 +36869,7 @@ if (__DEV__) {
       return root;
     }
 
-    var ReactVersion = "19.0.0-www-classic-9558ae95";
+    var ReactVersion = "19.0.0-www-classic-b18818ec";
 
     function createPortal$1(
       children,
@@ -50592,7 +50592,7 @@ if (__DEV__) {
 
     function createRoot$1(container, options) {
       {
-        if (!Internals.usingClientEntryPoint && !false) {
+        if (!Internals.usingClientEntryPoint) {
           error(
             'You are importing createRoot from "react-dom" which is not supported. ' +
               'You should instead import it from "react-dom/client".'
@@ -50605,7 +50605,7 @@ if (__DEV__) {
 
     function hydrateRoot$1(container, initialChildren, options) {
       {
-        if (!Internals.usingClientEntryPoint && !false) {
+        if (!Internals.usingClientEntryPoint) {
           error(
             'You are importing hydrateRoot from "react-dom" which is not supported. ' +
               'You should instead import it from "react-dom/client".'

compiled/facebook-www/ReactDOMTesting-dev.modern.js

@@ -46517,7 +46517,7 @@ if (__DEV__) {
       return root;
     }
 
-    var ReactVersion = "19.0.0-www-modern-4ad61d22";
+    var ReactVersion = "19.0.0-www-modern-f9d107af";
 
     function createPortal$1(
       children,
@@ -49077,7 +49077,7 @@ if (__DEV__) {
 
     function createRoot$1(container, options) {
       {
-        if (!Internals.usingClientEntryPoint && !false) {
+        if (!Internals.usingClientEntryPoint) {
           error(
             'You are importing createRoot from "react-dom" which is not supported. ' +
               'You should instead import it from "react-dom/client".'
@@ -49090,7 +49090,7 @@ if (__DEV__) {
 
     function hydrateRoot$1(container, initialChildren, options) {
       {
-        if (!Internals.usingClientEntryPoint && !false) {
+        if (!Internals.usingClientEntryPoint) {
           error(
             'You are importing hydrateRoot from "react-dom" which is not supported. ' +
               'You should instead import it from "react-dom/client".'