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.

scripts/rollup/shims/facebook-www/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;