Nix: Remove duplicate shell.nix

[ttrssreal]

If we really want we can introduce a compatibility layer, but flakes are pretty standard.

[AtkinsSJ]

I know nothing about Nix, but this only got added a couple of weeks ago in #3767. It feels a bit like every Nix user has a different way of doing things. 😅

cc: @NotAShelf Can we come to some kind of agreement for Nix?

[NotAShelf]

This is not about different ways of doing things, but about what "versions" of Nix would you like to support. shell.nix is an artifact of older, "legacy" (very heavy quotes here) nix2 and a flake.nix is the newer, shinier nix3. They are two different hills people to die on, but a majority of Nix consumers use flakes- the ideological arguments against flakes are mostly from the more technical part of the community. I, for one, welcome flakes, albeit with their flaws.

On this PR, I'm fully in favor. nix3 (flakes) enforces better idioms, and as @ttrssreal stated they are pretty standard. Removing the top-level shell.nix removes the legacy (nix2) interface, but retains the shell.nix I have added in the toolchain directory that is called from the flake. Should nix2 compatibility become a problem, there are clean solutions that we may choose to apply as necessary.

If there is anything specific you would like my input on, I am happy to provide.

[ttrssreal]

Don't mean to stir drama :3
Either way we shouldn't have 2 shell.nix's, like @NotAShelf says theres solutions if it becomes an issue