Drop support for dir-locals.nix, <nixpkgs>, etc.

In the spirit of Marie Kondo, I'm tidying up!

TL;DR:
- Prefer .envrc `use_nix` and delete all dir-locals.nix files
- Remove ~all references to <nixpkgs>, <unstable>, <depot> and prefer
  referencing each with briefcase.third_party.{pkgs,unstable,depot}
- Delete nixBufferFromShell function since I was only using that in
  dir-locals.nix files

utils/default.nix

@@ -11,10 +11,4 @@
   wrapNonNixProgram = { path, as }: pkgs.writeShellScriptBin as ''
     exec ${path} "$@"
   '';
-
-  # Expose the buildInputs from a Nix shell to an Emacs buffer. Intended to be
-  # called from dir-locals.nix files.
-  nixBufferFromShell = path: let
-    shell = import path;
-  in pkgs.nixBufferBuilders.withPackages shell.buildInputs;
 }