docs: Removed stale list entries
Removed two list entries in the design README that don't have a
corresponding document to link to.
Kata Containers design documents:
- [Kata Containers architecture](architecture.md)
-- Working scenarios of Kata Containers
- [API Design of Kata Containers](kata-api-design.md)
-- Kata Containers lifecycle
- [Design requirements for Kata Containers](kata-design-requirements.md)
