docs: Remove multi-backend-testing.md (moved to datajoint-docs)

The user-facing content from this developer spec has been incorporated
into datajoint-docs:
- Database backends specification (reference/specs/database-backends.md)
- PostgreSQL configuration guide (how-to/configure-database.md)

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

Author: dimitri-yatsenko