rebol/default.nix: remove unused file
rebol/default.nix: remove unused file

The rebol attribute has been removed over 4 years ago, so removing the
file as will is probably fine.

See d6cb238c8ec6b129d46d8281713e6f610ad9e45d.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs