doc: remove mention of the DMOZ catalog in ltree docs

Discussion: https://postgr.es/m/CAF4Au4xYem_W3KOuxcKct7=G4j8Z3uO9j3DUKTFJqUsfp_9pQg@mail.gmail.com

Author: Oleg Bartunov

Backpatch-through: 9.3

doc/src/sgml/ltree.sgml

@@ -32,9 +32,7 @@
    labels separated by dots, for example <literal>L1.L2.L3</>, representing
    a path from the root of a hierarchical tree to a particular node.  The
    length of a label path must be less than 65Kb, but keeping it under 2Kb is
-   preferable.  In practice this is not a major limitation; for example,
-   the longest label path in the DMOZ catalog (<ulink
-   url="http://www.dmoztools.net"></ulink>) is about 271 bytes.
+   preferable.
   </para>
 
   <para>