fix(nix): drop stale "vercel" group from #full variant

The `vercel` optional-dependency was removed from pyproject.toml in
#33067, but `nix/packages.nix` (added a few hours later in #33108)
still references `"vercel"` in the `#full` variant's
`extraDependencyGroups`. uv2nix fails evaluation with:

  error: Extra/group name 'vercel' does not match either extra or
  dependency group

Because `nix/devShell.nix` does
`inputsFrom = builtins.attrValues self'.packages`, the broken `#full`
derivation is pulled into the dev shell too, so `nix develop` /
direnv breaks on a fresh clone — not just `nix build .#full`.

nix/packages.nix

@@ -43,7 +43,6 @@
             "modal"
             "parallel-web"
             "tts-premium"
-            "vercel"
             "voice"
           ] ++ lib.optionals pkgs.stdenv.isLinux [ "matrix" ];
         };