Remove Nix integration

[yvan-sraka]

This PR modifies the cabal behavior. It's the follow-up to https://github.com/haskell/cabal/pull/9205, and completely removes the Nix integration feature. This PR should be merged only after a grace period during which the deprecation warning has been displayed to the user.

Checklist:

• [x] Patches conform to the coding conventions.
• [ ] Any changes that could be relevant to users have been recorded in the changelog.
• [x] The documentation has been updated, if necessary.
• [ ] Manual QA notes have been included.

Bonus points for added automated tests!

[ulysses4ever]

This looks like a good candidate for the next release: nix integration is a remnant from v1 times, and big nix users largely switched to v2 by now afaict

[zlonast]

@ulysses4ever can we recreate the pull request due to the author's inactivity?

[ulysses4ever]

@zlonast absolutely

[geekosaur]

For completeness: this PR has been replaced with #11191.