release-combined: Remove unused value `allSupportedNixpkgs`.

It was made obsolete by commit

    2de3caf0 -  nixos/release-combined.nix: List constituents of the 'tested' job by name

see also https://github.com/NixOS/nixpkgs/commit/f996744fd32f255b7fd33d9e83d9ef688097b367#r38642487.