docs: remove TODO.md in favor of issues

All unchecked items in TODO list are now replaced by issues in the
backlog: https://github.com/rackslab/racksdb/issues

- Tests of back references in Schema → #1
- Tests of default properties in Schema → #1
- Tests of boolean properties in Schema → #1
- Tests of circular dependencies in schemas → #1
- Tests of generic DB loads → #1
- Add class/object diagram of database schema in docs → #2
- Add JSON DBDumper → #3
- Draw pixel perfect lines → #4
- Draw racks based on imported SVG file → #5
- Draw equipments based on imported SVG file → #6
- Tiny web application to explore DB content → #7
- Check collisions in rooms → #8
- Check collisions in racks → #9
- Support additional defined types → #10
- Network cables and connections → #11
- IPAM → #12

The TODO.md file is not needed anymore.