emacs: remove outdated doc about package initialization

Since Emacs 27[1], there is no need to call package-initialize in the
init file.

[1]: https://git.savannah.gnu.org/cgit/emacs.git/tree/etc/NEWS.27?id=cae528457cb862dc886a34240c9d4c73035b6659#n227

doc/builders/packages/emacs.section.md

···
 {
   packageOverrides = pkgs: with pkgs; rec {
     myEmacsConfig = writeText "default.el" ''
-      ;; initialize package
-
-      (require 'package)
-      (package-initialize 'noactivate)
       (eval-when-compile
         (require 'use-package))
 

nixos/modules/services/editors/emacs.md

···
 
 ## Configuring Emacs {#module-services-emacs-configuring}
 
-The Emacs init file should be changed to load the extension packages at
-startup:
-
-::: {.example #module-services-emacs-package-initialisation}
-### Package initialization in `.emacs`
-
-```
-(require 'package)
-
-;; optional. makes unpure packages archives unavailable
-(setq package-archives nil)
-
-(setq package-enable-at-startup nil)
-(package-initialize)
-```
-:::
+If you want to only use extension packages from Nixpkgs, you can add
+`(setq package-archives nil)` to your init file.
 
 After the declarative Emacs package configuration has been tested,
 previously downloaded packages can be cleaned up by removing

pkgs/top-level/emacs-packages.nix

···
 # Alternative: use `emacs`, install everything to a system or user profile
 # and then add this at the start your `init.el`:
 /*
-  (require 'package)
-
-  ;; optional. makes unpure packages archives unavailable
-  (setq package-archives nil)
-
   ;; optional. use this if you install emacs packages to the system profile
   (add-to-list 'package-directory-list "/run/current-system/sw/share/emacs/site-lisp/elpa")
 
   ;; optional. use this if you install emacs packages to user profiles (with nix-env)
   (add-to-list 'package-directory-list "~/.nix-profile/share/emacs/site-lisp/elpa")
-
-  (package-initialize)
 */
 
 { pkgs'