Tweak documentation wording to avoid "pdfendlink" failure.

HEAD documentation was failing to build as US PDF for me, because a link
to "CREATE CAST" was getting split across pages.  Adjust wording to
remove this rather gratuitous cross-reference.

Author: tglsfdc

doc/src/sgml/catalogs.sgml

@@ -1427,8 +1427,7 @@
 
   <para>
    The catalog <structname>pg_cast</structname> stores data type conversion
-   paths, both built-in paths and those defined with
-   <xref linkend="sql-createcast">.
+   paths, both built-in and user-defined.
   </para>
 
   <para>