Remove docs mention of PGREALM variable

This variable was only used with Kerberos v4. That support was removed
in 2005, but we forgot to remove the documentation.

Noted by Shinichi Matsuda

doc/src/sgml/libpq.sgml

@@ -7009,22 +7009,6 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
      </para>
     </listitem>
 
-    <listitem>
-     <para>
-      <indexterm>
-       <primary><envar>PGREALM</envar></primary>
-      </indexterm>
-      <envar>PGREALM</envar> sets the Kerberos realm to use with
-      <productname>PostgreSQL</productname>, if  it is different from the
-      local realm.  If <envar>PGREALM</envar> is set,
-      <application>libpq</application> applications will attempt
-      authentication  with  servers for this realm and use separate ticket
-      files to avoid conflicts with local ticket files.   This
-      environment  variable is only used if GSSAPI authentication is
-      selected by the server.
-     </para>
-    </listitem>
-
     <listitem>
      <para>
       <indexterm>