nixosTests.lxd: remove references to removed test

pkgs/by-name/lx/lxc/package.nix

···
     tests = {
       incus-lts = nixosTests.incus-lts.container;
       lxc = nixosTests.lxc;
-      lxd = nixosTests.lxd.container;
     };
 
     updateScript = nix-update-script {

pkgs/os-specific/linux/nftables/default.nix

···
 
   passthru.tests = {
     inherit (nixosTests) firewall-nftables;
-    lxd-nftables = nixosTests.lxd.nftables;
     nat = { inherit (nixosTests.nat.nftables) firewall standalone; };
   };