[flags] Cleanup enableFragmentRefsScrollIntoView feature flag #37574

[!NOTE]
Stacked on #37573. That PR's commit shows up in the diff here until it lands; only the second commit is part of this change.

Summary

Inlines the enabled branch of enableFragmentRefsScrollIntoView and removes the flag from ReactFeatureFlags and all of its forks.

This is a DOM-only change. The flag's only consumer was ReactFiberConfigDOM, where it guarded the definition of FragmentInstance.prototype.scrollIntoView, so the code change is a plain unwrap and dedent of that one block with no logic edits.

The flag was declared false in the native forks, but those declarations were inert: Fabric never defined scrollIntoView on FragmentInstance at all, so removing the flag does not change any native behavior.

The one combined gate, @gate enableFragmentRefsTextNodes && enableFragmentRefsScrollIntoView, was reduced to @gate enableFragmentRefsTextNodes rather than dropped.

How did you test this change?

yarn lint, yarn prettier-check, and yarn flow for dom-node, dom-browser, and fabric all pass.

Full test suite run across experimental, stable, www-modern, www-classic, and xplat, with both --variant settings, plus --persistent. The remaining failures (Fizz / Flight / FrameScheduling / ClassEquivalence) are pre-existing: I diffed the individual failing test names against the parent commit and they are identical.