System: Move cachix configuration to mkCommonNixSettings library function

system/cachix.nix

@@ -1,16 +0,0 @@
-# WARN: this file will get overwritten by $ cachix use <name>
-{
-  pkgs,
-  lib,
-  ...
-}: let
-  folder = ./cachix;
-  toImport = name: value: folder + ("/" + name);
-  filterCaches = key: value: value == "regular" && lib.hasSuffix ".nix" key;
-  imports =
-    lib.mapAttrsToList toImport
-    (lib.filterAttrs filterCaches (builtins.readDir folder));
-in {
-  inherit imports;
-  nix.settings.substituters = ["https://cache.nixos.org/"];
-}

system/cachix/nix-community.nix

@@ -1,14 +0,0 @@
-{
-  nix.settings = {
-    substituters = [
-      "https://nix-community.cachix.org"
-      # "https://app.cachix.org/cache/nixos-rocm"
-      # "https://ai.cachix.org"
-    ];
-    trusted-public-keys = [
-      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
-      # "nixos-rocm.cachix.org-1:VEpsf7pRIijjd8csKjFNBGzkBqOmw8H9PRmgAq14LnE="
-      # "ai.cachix.org-1:N9dzRK+alWwoKXQlnn0H6aUx0lU/mspIoz8hMvGvbbc="
-    ];
-  };
-}