nix-tour: remove phases

pkgs/applications/misc/nix-tour/default.nix

···
     sha256 = "09b1vxli4zv1nhqnj6c0vrrl51gaira94i8l7ww96fixqxjgdwvb";
   };
 
-  phases = [ "unpackPhase" "installPhase" ];
-
   installPhase = ''
     mkdir -p $out/bin
     mkdir -p $out/share