Remove renderSubtreeIntoContainer_DO_NOT_USE (#28526)

Since D54648741 we no longer need this shim.

NOTE: There is still some work left to remove the
`unstable_renderSubtreeIntoContainer` export.

DiffTrain build for [eb33bd7477](https://github.com/facebook/react/commit/eb33bd74779df6e91c54a47e8f07b22b72a073c7)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-89021fb4ec9aa82194b0788566e736a4cedfc0e4
+eb33bd74779df6e91c54a47e8f07b22b72a073c7

compiled/facebook-www/shims/renderSubtreeIntoContainer_DO_NOT_USE.js

@@ -1,12 +0,0 @@
-/**
- * Copyright (c) Meta Platforms, Inc. and 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 ReactDOM = require('ReactDOM');
-
-module.exports = ReactDOM.unstable_renderSubtreeIntoContainer;