nix: Remove package definition

[WeetHet]

Let's be real. It's time to admit that having a nix package in the repo was an experiment and a failed one at that. After #21550, the build definitions were outdated for a month with no effort being made to improve the situation.

Nixpkgs has maintainers for the zed package willing to support building zed (only stable though), while trying to keep the build definitions in the repo up to date without team support is futile and will only create additional burden both on the zed team and the contributors, whose efforts would arguably be better spent working on different issues.

If someone really wants to run a nightly build of zed using nix, they could always do something similar to the approach I've tried in #22825. I've added an error pointing to it in place of the package with an intend to remove it later when it's reasonable to expect that anyone who is trying to use the package from the repo has seen it

A different solution would be for someone from the team to commit to supporting nightly builds using nix continuously, which I see as unlikely, seeing as that would probably slow down PRs and require contributors to know nix or for that team member to monitor PRs constantly

I've also taken the liberty to use nixpkgs-unstable ref for nixpkgs, as the dev shell is not nixos specific and thus updating nixpkgs should not be blocked on nixos-specific packages as well as updating to a maintained version of flake-compat

cc @SomeoneToIgnore @mrnugget

Release Notes:

• N/A

[maxdeviant]

I don’t see any harm in keeping them around, as they could serve as a useful starting point for someone who wants to help maintain them.