Remove POLY_TYPE_DESIGN.md from the migration branch

This type-erasure design doc is unrelated to the string-to-id migration; it
rode in on an unrelated commit. Untrack it here so the migration stays focused;
it will be upstreamed on its own.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rjoomen