emacs: Deprecate "guix-init.el".

'guix-init' served 2 purposes:

- to autoload guix commands (by requiring 'guix-autoloads');
- and to autoload Emacs packages installed with Guix (by requiring
  'guix-emacs').

The second purpose is not actual anymore, as Emacs package handles this
task now, so there is no need in 'guix-init' anymore.

* emacs/guix-init.el: Do not require 'guix-emacs'.  Add a deprecation
  message.
* doc/emacs.texi (Emacs Initial Setup): Recommend to use
  'guix-autoloads' instead of 'guix-init'.