[gtk-doc] README: remove SGML reference.

[Stefan Sauer <stefkost src gnome org>]

commit 868d9ba4f12aab6e37623a1fce1299908068b959
Author: Stefan Sauer <ensonic users sf net>
Date:   Tue Apr 16 07:26:34 2019 +0200

    README: remove SGML reference.
    
    We dropped this already.

 README | 2 --
 1 file changed, 2 deletions(-)
---
diff --git a/README b/README
index b67ff6c..00d10f3 100644
--- a/README
+++ b/README
@@ -19,8 +19,6 @@ The generated HTML documentation can be browsed in an ordinary web browser or
 by using the special Devhelp API browser
 (see https://wiki.gnome.org/Apps/Devhelp).
 
-Please use DoxBook XML instead of DocBook SGML - support for the later will be
-dropped.
 
 Requirements
 ============