books: add debian release

["der.hans" <Verteiler+debian-www@LuftHans.com>]

moin moin,

Attached is a patch to add a new tag for debian release in doc/books and add
debian release to the first listed book.

ciao,

der.hans
--
#  https://www.LuftHans.com   https://www.PhxLinux.org
#  Hope has two beautiful daughters: Anger and Courage. Anger at the way
#  things are, and Courage to struggle to create things as they should be.
#  -- St. Augustine

diff --git a/english/doc/books.data b/english/doc/books.data
index 0f709f5816b..fbb058bf81b 100644
--- a/english/doc/books.data
+++ b/english/doc/books.data
@@ -23,7 +23,6 @@
 <bookentry>
   <title Debian 9, van inleiding tot systeembeheer>
   <author Koen Wybo>
-  <debian-release "9/Stretch">
   <publisher pumbo>
   <URL "http://linuxleren.nl/";>
   <email koen.wybo@telenet.be>
diff --git a/english/doc/books.def b/english/doc/books.def
index b6685ad17ad..510eec8279d 100644
--- a/english/doc/books.def
+++ b/english/doc/books.def
@@ -37,9 +37,6 @@
 <define-tag author>
   <strong><gettext domain="doc">Author:</gettext></strong> %attributes<br />
 </define-tag>
-<define-tag debian-release>
-  <strong><gettext domain="doc">Debian Release:</gettext></strong> %attributes<br />
-</define-tag>
 <define-tag email>
   <strong><gettext domain="doc">email:</gettext></strong> 
   <a href="mailto:%attributes";>%attributes</a><br />