Remove all custom server integration examples (#42549)
Fixes @leerob's comment https://github.com/vercel/next.js/pull/42110#issuecomment-1304897491
~~Deprecated~~ Removed all custom server integration examples (Express, Fastify, etc.) in favour of the main `custom-server` example, ~~and left behind a helpful `README.md` in case they might be linked to.~~
## Documentation / Examples
- [X] Make sure the linting passes by running `pnpm build && pnpm lint`
- [X] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
