Remove createRootStrictEffectsByDefault flag (#28102)

There's no need to separate strict mode from strict effects mode any
more.

I didn't clean up the `StrictEffectMode` fiber flag, because it's used
to prevent strict effects in legacy mode. I could replace those checks
with `LegacyMode` checks, but when we remove legacy mode, we can remove
that flag and condense them into one StrictMode flag away.

Author: rickhanlonii

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -776,14 +776,7 @@ describe('ReactDOMServerPartialHydration', () => {
     const span2 = container.getElementsByTagName('span')[0];
     // This is a new node.
     expect(span).not.toBe(span2);
-
-    if (gate(flags => flags.dfsEffectsRefactor)) {
-      // The effects list refactor causes this to be null because the Suspense Activity's child
-      // is null. However, since we can't hydrate Suspense in legacy this change in behavior is ok
-      expect(ref.current).toBe(null);
-    } else {
-      expect(ref.current).toBe(span2);
-    }
+    expect(ref.current).toBe(null);
 
     // Resolving the promise should render the final content.
     suspend = false;

packages/react-reconciler/src/ReactFiber.js

@@ -28,7 +28,6 @@ import {
   isHostSingletonType,
 } from './ReactFiberConfig';
 import {
-  createRootStrictEffectsByDefault,
   enableCache,
   enableProfilerTimer,
   enableScopeAPI,
@@ -456,7 +455,7 @@ export function createHostRootFiber(
   let mode;
   if (tag === ConcurrentRoot) {
     mode = ConcurrentMode;
-    if (isStrictMode === true || createRootStrictEffectsByDefault) {
+    if (isStrictMode === true) {
       mode |= StrictLegacyMode | StrictEffectsMode;
     }
     if (