Cleanup alwaysThrottleDisappearingFallbacks Flag (#28639)

## Summary

After realizing that this feature flag is entangled with
`alwaysThrottleRetries`, we're going to undo
https://github.com/facebook/react/pull/28550

## How did you test this change?

```
$ yarn test
$ yarn flow dom-browser
$ yarn flow dom-fb
$ yarn flow fabric
```

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -40,7 +40,7 @@ import type {
 } from './ReactFiberTracingMarkerComponent';
 
 import {
-  alwaysThrottleDisappearingFallbacks,
+  alwaysThrottleRetries,
   enableCreateEventHandleAPI,
   enableProfilerTimer,
   enableProfilerCommitHooks,
@@ -2893,7 +2893,7 @@ function commitMutationEffectsOnFiber(
           current !== null &&
           (current.memoizedState: SuspenseState | null) !== null;
 
-        if (alwaysThrottleDisappearingFallbacks) {
+        if (alwaysThrottleRetries) {
           if (isShowingFallback !== wasShowingFallback) {
             // A fallback is either appearing or disappearing.
             markCommitTimeOfFallback();

packages/shared/ReactFeatureFlags.js

@@ -110,8 +110,6 @@ export const enableUseEffectEventHook = __EXPERIMENTAL__;
 // (handled with an MutationObserver) instead of inline-scripts
 export const enableFizzExternalRuntime = true;
 
-export const alwaysThrottleDisappearingFallbacks = true;
-
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.native-fb-dynamic.js

@@ -17,7 +17,6 @@
 // flag here but it won't be set to `true` in any of our test runs. Need to
 // add a test configuration for React Native.
 
-export const alwaysThrottleDisappearingFallbacks = __VARIANT__;
 export const alwaysThrottleRetries = __VARIANT__;
 export const consoleManagedByDevToolsDuringStrictMode = __VARIANT__;
 export const enableAsyncActions = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -19,7 +19,6 @@ const dynamicFlags: DynamicExportsType = (dynamicFlagsUntyped: any);
 // We destructure each value before re-exporting to avoid a dynamic look-up on
 // the exports object every time a flag is read.
 export const {
-  alwaysThrottleDisappearingFallbacks,
   alwaysThrottleRetries,
   consoleManagedByDevToolsDuringStrictMode,
   enableAsyncActions,

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -27,7 +27,6 @@ export const enableReactTestRendererWarning = __TODO_NEXT_RN_MAJOR__;
 export const enableAsyncActions = __TODO_NEXT_RN_MAJOR__;
 export const consoleManagedByDevToolsDuringStrictMode = __TODO_NEXT_RN_MAJOR__;
 export const enableDeferRootSchedulingToMicrotask = __TODO_NEXT_RN_MAJOR__;
-export const alwaysThrottleDisappearingFallbacks = __TODO_NEXT_RN_MAJOR__;
 export const alwaysThrottleRetries = __TODO_NEXT_RN_MAJOR__;
 export const enableInfiniteRenderLoopDetection = __TODO_NEXT_RN_MAJOR__;
 export const enableComponentStackLocations = __TODO_NEXT_RN_MAJOR__;

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -72,7 +72,6 @@ export const enableDeferRootSchedulingToMicrotask = true;
 
 export const enableAsyncActions = true;
 
-export const alwaysThrottleDisappearingFallbacks = true;
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.native.js

@@ -74,7 +74,6 @@ export const enableDeferRootSchedulingToMicrotask = false;
 
 export const enableAsyncActions = true;
 
-export const alwaysThrottleDisappearingFallbacks = true;
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -75,7 +75,6 @@ export const enableDeferRootSchedulingToMicrotask = true;
 
 export const enableAsyncActions = true;
 
-export const alwaysThrottleDisappearingFallbacks = false;
 export const alwaysThrottleRetries = true;
 
 export const passChildrenWhenCloningPersistedNodes = false;

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -24,7 +24,6 @@ export const forceConcurrentByDefaultForTesting = __VARIANT__;
 export const enableUnifiedSyncLane = __VARIANT__;
 export const enableTransitionTracing = __VARIANT__;
 export const enableDeferRootSchedulingToMicrotask = __VARIANT__;
-export const alwaysThrottleDisappearingFallbacks = __VARIANT__;
 export const alwaysThrottleRetries = __VARIANT__;
 export const enableDO_NOT_USE_disableStrictPassiveEffect = __VARIANT__;
 export const enableUseDeferredValueInitialArg = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -27,7 +27,6 @@ export const {
   enableRetryLaneExpiration,
   enableTransitionTracing,
   enableDeferRootSchedulingToMicrotask,
-  alwaysThrottleDisappearingFallbacks,
   alwaysThrottleRetries,
   enableDO_NOT_USE_disableStrictPassiveEffect,
   disableSchedulerTimeoutInWorkLoop,