chore: remove unused viewer_migration_example.py

Delete dead code file that was only self-referential (example/documentation).
Not imported or used by any other module.

Verified safe to delete:
- Not imported by cli.py
- Not imported by any other file in openadapt_ml/
- Only referenced itself in docstring and output filename

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: abrichr