[nixos] Update recommended method for disabling nixos-company-options

Per discussion in #13164, we have a standard, consistent technique for doing
this across any layer. Provided that `global-company-mode` is not set, this
works as expected to disable company without requiring custom variables.

CHANGELOG.develop

@@ -2465,8 +2465,6 @@ Other:
   function (thanks to Profpatsch)
 - Associate nix-mode with .nix files (thanks to jpathy)
 - Updated layer banner (thanks to kalium)
-- Added option =nixos-enable-company= to toggle company auto-completion
-  (thanks to bhipple)
 **** Nim
 - Added key binding ~SPC m h h~ to show symbol documentation
   (thanks to Valts Liepiņš)

layers/+os/nixos/README.org

@@ -28,12 +28,8 @@ add =nixos= to the existing =dotspacemacs-configuration-layers= list in this
 file.
 
 * Configuration
-Toggle whether =company-nixos-options= completion is enabled (defaults to =t=).
-
-#+BEGIN_SRC emacs-lisp
-  (setq-default dotspacemacs-configuration-layers
-                '((nixos :variables nixos-enable-company t)))
-#+END_SRC
+On some systems, =company-nixos-options= may be very slow. If this is the case,
+see the section on disabling the [[https://github.com/syl20bnr/spacemacs/blob/develop/doc/DOCUMENTATION.org#disabling-layer-services-in-other-layers][disabling auto-complete]] for the =nixos= layer.
 
 * Key bindings
 ** NixOS Options

layers/+os/nixos/config.el

@@ -1,14 +0,0 @@
-;;; config.el --- NixOS Layer configuration File for Spacemacs
-;;
-;; Copyright (c) 2015-2020 Sylvain Benner & Contributors
-;;
-;; Author: Sylvain Benner <sylvain.benner@gmail.com>
-;; URL: https://github.com/syl20bnr/spacemacs
-;;
-;; This file is not part of GNU Emacs.
-;;
-;;; License: GPLv3
-
-
-(defvar nixos-enable-company t
-  "Enable company completion via company-nixos-options")

layers/+os/nixos/packages.el

@@ -10,26 +10,23 @@
 ;;; License: GPLv3
 
 (defconst nixos-packages
-      '(company
-        (company-nixos-options :requires company)
-        flycheck
-        (helm-nixos-options :requires helm)
-        nix-mode
-        nixos-options))
+  '((company-nixos-options :requires company)
+     flycheck
+     (helm-nixos-options :requires helm)
+     nix-mode
+     nixos-options))
 
 (defun nixos/post-init-company ()
-  (when nixos-enable-company
-    (let ((backends '(company-capf)))
-      (when (configuration-layer/package-used-p 'company-nixos-options)
-        (add-to-list 'backends 'company-nixos-options t))
-      (eval `(spacemacs|add-company-backends
-               :backends ,backends
-               :modes nix-mode)))))
+  (let ((backends '(company-capf)))
+    (when (configuration-layer/package-used-p 'company-nixos-options)
+      (add-to-list 'backends 'company-nixos-options t))
+    (eval `(spacemacs|add-company-backends
+              :backends ,backends
+              :modes nix-mode))))
 
 (defun nixos/init-company-nixos-options ()
- (use-package company-nixos-options
-   :if nixos-enable-company
-   :defer t))
+  (use-package company-nixos-options
+    :defer t))
 
 (defun nixos/init-helm-nixos-options ()
   (use-package helm-nixos-options