fix: remove unused docutils import from dsm_util.py

`relative_path` was imported but never used; it added a hard runtime
dependency on docutils that broke `dsm_util.py check` in environments
without it (notably the system python the JetBrains plugin falls back
to when no venv is configured).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
