Remove withContext from top-level API

Author: zpao

src/browser/ui/React.js

@@ -75,7 +75,6 @@ var React = {
   renderToStaticMarkup: ReactServerRendering.renderToStaticMarkup,
   unmountComponentAtNode: ReactMount.unmountComponentAtNode,
   isValidElement: ReactElement.isValidElement,
-  withContext: ReactContext.withContext,
 
   // Hook for JSX spread, don't use this for anything else.
   __spread: assign