Remove FB specific build (#17875)

I moved unstable_SuspenseList internally. We don't need the FB build.
I plan on also removing the ReactDOM fb specific entry. We shouldn't add
any more FB specific internals nor APIs. If they're experimental they
should go onto the experimental builds to avoid too many permutations.

packages/react/index.fb.js

@@ -1,14 +0,0 @@
-/**
- * Copyright (c) Facebook, Inc. and its affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- */
-
-'use strict';
-
-const ReactFB = require('./src/ReactFB');
-
-// TODO: decide on the top-level export form.
-// This is hacky but makes it work with both Rollup and Jest.
-module.exports = ReactFB.default || ReactFB;

packages/react/src/ReactFB.js

@@ -1,15 +0,0 @@
-/**
- * Copyright (c) Facebook, Inc. and its affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @flow
- */
-
-import React from './React';
-
-// TODO: Temporary alias until we update the callers downstream.
-React.unstable_SuspenseList = React.SuspenseList;
-
-export default React;