Remove next-dev references and benchmarks (#6026)
We've rearchitected Next.js+Turbopack so Turbopack does not run
reimplement pieces of Next.js in its devserver. This:
- Removes references and tests of the `next-dev` binary, which is no
longer reachable through `next --turbo`. Uses `next-swc` in its place
when testing.
- 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-1653
