Remove next-dev and its test suite and benchmarks (#55983)
We've rearchitected Next.js+Turbopack so Turbopack does not run
reimplement pieces of Next.js in its devserver. This:
- Removes the `next-dev` binary, which is no longer reachable through
`next --turbo`.
- Removes its test suite, as much of it is tested (and often more
thoroughly) by the Next.js test suite
- Removes its benchmark suite, which should be covered by
`Turbopack-bench` by
https://github.com/vercel/turbo/tree/main/crates/turbopack-bench
Test Plan: CI
Closes WEB-1652
