Remove legacy hydration mode (#28440)

While Meta is still using legacy mode and we can't remove completely,
Meta is not using legacy hydration so we should be able to remove that.

This is just the first step. Once removed, we can vastly simplify the
DOMConfig for hydration.

This will have to be rebased when tests are upgraded.

DiffTrain build for [670d61b](670d61b)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-dbfbfb3312db019183ef92fd2ef110cc7d807e80
+670d61bea23470e980ba13c1c8441e375779b0b8

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

@@ -66,7 +66,7 @@ if (__DEV__) {
       return self;
     }
 
-    var ReactVersion = "19.0.0-www-classic-36e84f9d";
+    var ReactVersion = "19.0.0-www-classic-841735f0";
 
     var LegacyRoot = 0;
     var ConcurrentRoot = 1;
@@ -16852,15 +16852,6 @@ if (__DEV__) {
         // This is the first render pass. Attempt to hydrate.
         pushPrimaryTreeSuspenseHandler(workInProgress); // We should never be hydrating at this point because it is the first pass,
 
-        if ((workInProgress.mode & ConcurrentMode) === NoMode) {
-          return retrySuspenseComponentWithoutHydrating(
-            current,
-            workInProgress,
-            renderLanes,
-            null
-          );
-        }
-
         if (isSuspenseInstanceFallback()) {
           // This boundary is in a permanent fallback state. In this case, we'll never
           // get an update and we'll never be able to hydrate the final content. Let's just try the

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

@@ -66,7 +66,7 @@ if (__DEV__) {
       return self;
     }
 
-    var ReactVersion = "19.0.0-www-modern-2edfe4d7";
+    var ReactVersion = "19.0.0-www-modern-37e2c118";
 
     var LegacyRoot = 0;
     var ConcurrentRoot = 1;
@@ -16546,15 +16546,6 @@ if (__DEV__) {
         // This is the first render pass. Attempt to hydrate.
         pushPrimaryTreeSuspenseHandler(workInProgress); // We should never be hydrating at this point because it is the first pass,
 
-        if ((workInProgress.mode & ConcurrentMode) === NoMode) {
-          return retrySuspenseComponentWithoutHydrating(
-            current,
-            workInProgress,
-            renderLanes,
-            null
-          );
-        }
-
         if (isSuspenseInstanceFallback()) {
           // This boundary is in a permanent fallback state. In this case, we'll never
           // get an update and we'll never be able to hydrate the final content. Let's just try the