flake: remove the 'follows' directive (#2218)

nixFlake suffers from a bug when using "follows" keyword:
NixOS/nix#3602

Author: teto

flake.nix

@@ -16,8 +16,6 @@
     flake-utils.url = "github:numtide/flake-utils";
     pre-commit-hooks = {
       url = "github:cachix/pre-commit-hooks.nix";
-      inputs.nixpkgs.follows = "nixpkgs";
-      inputs.flake-utils.follows = "flake-utils";
     };
     gitignore = {
       url = "github:hercules-ci/gitignore.nix";