nixos/emacs: remove docbook section in emacs documentation

Since with the completion of the docbook migration) it seems unclear
what relevance editing xml in generall and docbook in particular with
Emacs still has to NixOS at all, and people interested in the topic
will presumably look to other resources elsewhere (e.g. to the nXML
mode's actual documenation).

nixos/modules/services/editors/emacs.md

@@ -366,44 +366,3 @@ convenient if you regularly edit Nix files.
 You can use `woman` to get completion of all available
 man pages. For example, type `M-x woman <RET> nixos-rebuild <RET>.`
 
-### Editing DocBook 5 XML Documents {#sec-emacs-docbook-xml}
-
-Emacs includes
-[nXML](https://www.gnu.org/software/emacs/manual/html_node/nxml-mode/Introduction.html),
-a major-mode for validating and editing XML documents. When editing DocBook
-5.0 documents, such as [this one](#book-nixos-manual),
-nXML needs to be configured with the relevant schema, which is not
-included.
-
-To install the DocBook 5.0 schemas, either add
-{var}`pkgs.docbook5` to [](#opt-environment.systemPackages)
-([NixOS](#sec-declarative-package-mgmt)), or run
-`nix-env -f '<nixpkgs>' -iA docbook5`
-([Nix](#sec-ad-hoc-packages)).
-
-Then customize the variable {var}`rng-schema-locating-files` to
-include {file}`~/.emacs.d/schemas.xml` and put the following
-text into that file:
-::: {.example #ex-emacs-docbook-xml}
-### nXML Schema Configuration (`~/.emacs.d/schemas.xml`)
-
-```xml
-<?xml version="1.0"?>
-<!--
-  To let emacs find this file, evaluate:
-  (add-to-list 'rng-schema-locating-files "~/.emacs.d/schemas.xml")
--->
-<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
-  <!--
-    Use this variation if pkgs.docbook5 is added to environment.systemPackages
-  -->
-  <namespace ns="http://docbook.org/ns/docbook"
-             uri="/run/current-system/sw/share/xml/docbook-5.0/rng/docbookxi.rnc"/>
-  <!--
-    Use this variation if installing schema with "nix-env -iA pkgs.docbook5".
-  <namespace ns="http://docbook.org/ns/docbook"
-             uri="../.nix-profile/share/xml/docbook-5.0/rng/docbookxi.rnc"/>
-  -->
-</locatingRules>
-```
-:::