Remove createMutableSource from stable exports (#23352)

I removed useMutableSource in a previous PR but forgot this one.

We still export it in the FB builds until we can migrate the internal
callers (Recoil).

Author: acdlite

packages/react/index.stable.js

@@ -21,7 +21,6 @@ export {
   createContext,
   createElement,
   createFactory,
-  createMutableSource as unstable_createMutableSource,
   createRef,
   forwardRef,
   isValidElement,