Remove FeatureFlags fork for react-dom/unstable_testing (#26383)

This doesn't need its own set of flags. We use things like `__PROFILE__`
in the regular feature flags file to fork for the `react-dom/profiling`
build so we can do the same here if needed but I don't think we actually
need to fork this anywhere as far as I can tell.

scripts/rollup/forks.js

@@ -135,14 +135,6 @@ const forks = Object.freeze({
  return './packages/shared/forks/ReactFeatureFlags.test-renderer.www.js';
  }
  return './packages/shared/forks/ReactFeatureFlags.test-renderer.js';
- case 'react-dom/unstable_testing':
- switch (bundleType) {
- case FB_WWW_DEV:
- case FB_WWW_PROD:
- case FB_WWW_PROFILING:
- return './packages/shared/forks/ReactFeatureFlags.testing.www.js';
- }
- return './packages/shared/forks/ReactFeatureFlags.testing.js';
  default:
  switch (bundleType) {
  case FB_WWW_DEV: