Remove codesandbox-ci

codesandbox-ci only supports Node.js 18.13 which is not compatible with ESLint v9.

We don't need ESLint in csb-ci but it makes the install process more complicated just for csb-ci.

We have https://react-builds.vercel.app/ for automatically preparing builds from PRs anyway so all codesandbox-ci does is give us premade codesandboxes.
We rarely use these anyway so it isn't that useful after all.

Author: eps1lon