chore: Make remote runtime class default to None

[xingyaoww]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

---

Give a summary of what the PR does, explaining any non-trivial design decisions

We can leave config to use gvisor for default, then try to customize it to sysbox in the cloud version via env var.

---

Link of any specific issues this addresses

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:3733cc0-nikolaik   --name openhands-app-3733cc0   docker.all-hands.dev/all-hands-ai/openhands:3733cc0