Revert "Set up cachix as emacs-overlay binary cache"

This reverts commit 1223542226.

modules/cachix.nix

@@ -1,13 +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.binaryCaches = [ "https://cache.nixos.org/" ];
-}