docs: remove stale standalone RELEASE_NOTES_2.1.0.md

Unreferenced leftover (4 releases behind); CHANGELOG.md is the single source
of release history and GitHub Releases carry the per-version notes. Keeping a
top-level file named for an old version is misleading to anyone browsing the repo.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: fgapel