Remove checkReact (#21132)

I don't know what this is useful for but I suspect it was only useful at
FB and is not applicable to ES modules at FB nor elsewhere.

Author: sebmarkbage

packages/react-dom/src/client/ReactDOM.js

@@ -10,7 +10,6 @@
 import type {ReactNodeList} from 'shared/ReactTypes';
 import type {Container} from './ReactDOMHostConfig';
 
-import '../shared/checkReact';
 import {
   findDOMNode,
   render,