Remove migration scripts from git tracking

Removed from git (but kept locally):
- scripts/migrate_aesara_refs.py
- scripts/migrate_at_to_pt.py
- scripts/migrate_imports.py

These scripts were used during the PyMC v5 migration and are no longer needed in version control.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: twiecki