nixos/stage-1: remove dead code
nixos/stage-1: remove dead code

This special case for Btrfs was added in 51bc82960a23d2db786465f2deb709276ee3fbe8. One year later beddd36c953f98e95164bdff20b0a6ead61ad71c added code to skip the fsck entirely if the filesystem is Btrfs. This made the `if` statement unnecessary.

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