Remove Emacs interface.

Remove Emacs interface.

* emacs/guix-about.el: Remove file.
* emacs/guix-backend.el: Likewise.
* emacs/guix-base.el: Likewise.
* emacs/guix-buffer.el: Likewise.
* emacs/guix-build-log.el: Likewise.
* emacs/guix-command.el: Likewise.
* emacs/guix-config.el.in: Likewise.
* emacs/guix-devel.el: Likewise.
* emacs/guix-entry.el: Likewise.
* emacs/guix-external.el: Likewise.
* emacs/guix-geiser.el: Likewise.
* emacs/guix-guile.el: Likewise.
* emacs/guix-help-vars.el: Likewise.
* emacs/guix-helper.scm.in: Likewise.
* emacs/guix-history.el: Likewise.
* emacs/guix-hydra-build.el: Likewise.
* emacs/guix-hydra-jobset.el: Likewise.
* emacs/guix-hydra.el: Likewise.
* emacs/guix-info.el: Likewise.
* emacs/guix-init.el: Likewise.
* emacs/guix-license.el: Likewise.
* emacs/guix-list.el: Likewise.
* emacs/guix-location.el: Likewise.
* emacs/guix-main.scm: Likewise.
* emacs/guix-messages.el: Likewise.
* emacs/guix-pcomplete.el: Likewise.
* emacs/guix-popup.el: Likewise.
* emacs/guix-prettify.el: Likewise.
* emacs/guix-profiles.el: Likewise.
* emacs/guix-read.el: Likewise.
* emacs/guix-ui-generation.el: Likewise.
* emacs/guix-ui-license.el: Likewise.
* emacs/guix-ui-location.el: Likewise.
* emacs/guix-ui-package.el: Likewise.
* emacs/guix-ui-system-generation.el: Likewise.
* emacs/guix-ui.el: Likewise.
* emacs/guix-utils.el: Likewise.
* emacs/local.mk: Likewise.
* doc/emacs.texi: Likewise.
* doc/guix.texi: Remove cross-references to Emacs nodes.
(Package Management): Mention 'emacs-guix' package.
* doc/contributing.texi (The Perfect Setup): Remove the reference.
* doc/htmlxref.cnf: Add 'emacs-guix' URL.
* Makefile.am: Remove Emacs stuff.
* configure.ac: Likewise.
* gnu/packages/package-management.scm (guix-0.12.0)[native-inputs]:
Remove "emacs".
[propagated-inputs]: Remove "geiser" and "emacs-magit-popup".

Co-authored-by: Ludovic Courtès <ludo@gnu.org>