flake: remove compatibility with nix < 2.7 layout

flake.nix

@@ -11,10 +11,7 @@
 
  outputs = { self, nixpkgs, flake-utils }: let
  inherit (nixpkgs.lib)
- elem filterAttrs head mapAttrs mapAttrs' optionalAttrs replaceStrings warnIf;
-
- inherit (builtins)
- nixVersion compareVersions;
+ elem filterAttrs head mapAttrs' optionalAttrs replaceStrings;
 
  overlay = import ./.;
 
@@ -38,18 +35,6 @@
  default = overlay;
  rust-overlay = overlay;
  };
-
- # Compatible fields.
- # Put them outside `eachSystem` to suppress duplicated warnings.
- overlay =
- warnIf (compareVersions nixVersion "2.7" >= 0)
- "rust-overlay's flake output `overlay` is deprecated in favor of `overlays.default` for Nix >= 2.7"
- overlay;
- defaultPackage =
- warnIf (compareVersions nixVersion "2.7" >= 0)
- "rust-overlay's flake output `defaultPackage.<system>` is deprecated in favor of `packages.<system>.default` for Nix >= 2.7"
- (mapAttrs (_: pkgs: pkgs.default) self.packages);
-
  } // flake-utils.lib.eachSystem allSystems (system: let
  pkgs = import nixpkgs { inherit system; overlays = [ overlay ]; };
  in {