Clean up enablePostpone Experiment (facebook#35048)

We're not shipping this and it's a lot of code to maintain that is
blocking my refactor of Fizz for SuspenseList.

Author: sebmarkbage

packages/react-client/src/ReactFlightClient.js

@@ -39,12 +39,9 @@ import type {
   EncodeFormActionCallback,
 } from './ReactFlightReplyClient';
 
-import type {Postpone} from 'react/src/ReactPostpone';
-
 import type {TemporaryReferenceSet} from './ReactFlightTemporaryReferences';
 
 import {
-  enablePostpone,
   enableProfilerTimer,
   enableComponentPerformanceTrack,
   enableAsyncDebugInfo,
@@ -89,7 +86,6 @@ import {
 import {
   REACT_LAZY_TYPE,
   REACT_ELEMENT_TYPE,
-  REACT_POSTPONE_TYPE,
   ASYNC_ITERATOR,
   REACT_FRAGMENT_TYPE,
 } from 'shared/ReactSymbols';
@@ -3460,88 +3456,6 @@ function resolveErrorDev(
   return error;
 }
 
-function resolvePostponeProd(
-  response: Response,
-  id: number,
-  streamState: StreamState,
-): void {
-  if (__DEV__) {
-    // These errors should never make it into a build so we don't need to encode them in codes.json
-    // eslint-disable-next-line react-internal/prod-error-codes
-    throw new Error(
-      'resolvePostponeProd should never be called in development mode. Use resolvePostponeDev instead. This is a bug in React.',
-    );
-  }
-  const error = new Error(
-    'A Server Component was postponed. The reason is omitted in production' +
-      ' builds to avoid leaking sensitive details.',
-  );
-  const postponeInstance: Postpone = (error: any);
-  postponeInstance.$$typeof = REACT_POSTPONE_TYPE;
-  postponeInstance.stack = 'Error: ' + error.message;
-  const chunks = response._chunks;
-  const chunk = chunks.get(id);
-  if (!chunk) {
-    const newChunk: ErroredChunk<any> = createErrorChunk(
-      response,
-      postponeInstance,
-    );
-    chunks.set(id, newChunk);
-  } else {
-    triggerErrorOnChunk(response, chunk, postponeInstance);
-  }
-}
-
-function resolvePostponeDev(
-  response: Response,
-  id: number,
-  reason: string,
-  stack: ReactStackTrace,
-  env: string,
-  streamState: StreamState,
-): void {
-  if (!__DEV__) {
-    // These errors should never make it into a build so we don't need to encode them in codes.json
-    // eslint-disable-next-line react-internal/prod-error-codes
-    throw new Error(
-      'resolvePostponeDev should never be called in production mode. Use resolvePostponeProd instead. This is a bug in React.',
-    );
-  }
-  let postponeInstance: Postpone;
-  const callStack = buildFakeCallStack(
-    response,
-    stack,
-    env,
-    false,
-    // $FlowFixMe[incompatible-use]
-    Error.bind(null, reason || ''),
-  );
-  const rootTask = response._debugRootTask;
-  if (rootTask != null) {
-    postponeInstance = rootTask.run(callStack);
-  } else {
-    postponeInstance = callStack();
-  }
-  postponeInstance.$$typeof = REACT_POSTPONE_TYPE;
-  const chunks = response._chunks;
-  const chunk = chunks.get(id);
-  if (!chunk) {
-    const newChunk: ErroredChunk<any> = createErrorChunk(
-      response,
-      postponeInstance,
-    );
-    if (__DEV__) {
-      resolveChunkDebugInfo(response, streamState, newChunk);
-    }
-    chunks.set(id, newChunk);
-  } else {
-    if (__DEV__) {
-      resolveChunkDebugInfo(response, streamState, chunk);
-    }
-    triggerErrorOnChunk(response, chunk, postponeInstance);
-  }
-}
-
 function resolveErrorModel(
   response: Response,
   id: number,
@@ -4893,25 +4807,6 @@ function processFullStringRow(
       return;
     }
     // Fallthrough
-    case 80 /* "P" */: {
-      if (enablePostpone) {
-        if (__DEV__) {
-          const postponeInfo = JSON.parse(row);
-          resolvePostponeDev(
-            response,
-            id,
-            postponeInfo.reason,
-            postponeInfo.stack,
-            postponeInfo.env,
-            streamState,
-          );
-        } else {
-          resolvePostponeProd(response, id, streamState);
-        }
-        return;
-      }
-    }
-    // Fallthrough
     default: /* """ "{" "[" "t" "f" "n" "0" - "9" */ {
       if (__DEV__ && row === '') {
         resolveDebugHalt(response, id);

packages/react-dom/src/__tests__/ReactDOMFizzDeferredValue-test.js

@@ -90,43 +90,6 @@ describe('ReactDOMFizzForm', () => {
     expect(container.textContent).toEqual('Final');
   });
 
-  // @gate enablePostpone
-  it(
-    'if initial value postpones during hydration, it will switch to the ' +
-      'final value instead',
-    async () => {
-      function Content() {
-        const isInitial = useDeferredValue(false, true);
-        if (isInitial) {
-          React.unstable_postpone();
-        }
-        return <Text text="Final" />;
-      }
-
-      function App() {
-        return (
-          <div>
-            <Suspense fallback={<Text text="Loading..." />}>
-              <Content />
-            </Suspense>
-          </div>
-        );
-      }
-
-      const stream = await serverAct(() =>
-        ReactDOMServer.renderToReadableStream(<App />),
-      );
-      await readIntoContainer(stream);
-      expect(container.textContent).toEqual('Loading...');
-
-      assertLog(['Loading...']);
-      // After hydration, it's updated to the final value
-      await act(() => ReactDOMClient.hydrateRoot(container, <App />));
-      expect(container.textContent).toEqual('Final');
-      assertLog(['Loading...', 'Final']);
-    },
-  );
-
   it(
     'useDeferredValue during hydration has higher priority than remaining ' +
       'incremental hydration',