Remove reference.ced

This is a parsed DocBook DTD for the PSGML Emacs mode, but it hasn't
been updated since we switched to DocBook 4.2 about seven years ago.
Also, PSGML has deprecated this method of DTD parsing.

doc/src/sgml/docguide.sgml

@@ -1005,10 +1005,8 @@ save_size.pdfjadetex = 15000
 (setq sgml-indent-step 1)
 (setq sgml-indent-data t)
 (setq sgml-parent-document nil)
-(setq sgml-default-dtd-file "./reference.ced")
 (setq sgml-exposed-tags nil)
 (setq sgml-catalog-files '("/usr/local/share/sgml/catalog"))
-(setq sgml-ecat-files nil)
 
 (autoload 'sgml-mode "psgml" "Major mode to edit SGML files." t )
 </programlisting>
@@ -1025,8 +1023,6 @@ save_size.pdfjadetex = 15000
     </para>
 
     <para>
-     The <productname>PostgreSQL</productname> distribution includes a
-     parsed DTD definitions file <filename>reference.ced</filename>.
      You might find that when using <productname>PSGML</productname>, a
      comfortable way of working with these separate files of book
      parts is to insert a proper <literal>DOCTYPE</literal>
@@ -1056,6 +1052,8 @@ save_size.pdfjadetex = 15000
      <productname>PSGML</productname>, but it's less confusing and
      lighter weight.  Also, it offers syntax highlighting (font lock),
      which can be very helpful.
+     <filename>src/tools/editors/emacs.samples</filename> contains
+     sample settings for this mode.
     </para>
 
     <para>