output/mvp: remove obsolete plugin

The hardware is obsolete, and the product does not exist anymore on
the Hauppauge web site.  Let's see if anybody complains about the
removal.

doc/user.xml

@@ -1492,16 +1492,6 @@ systemctl start mpd.socket</programlisting>
         </informaltable>
       </section>
 
-      <section>
-        <title><varname>mvp</varname></title>
-
-        <para>
-          The <varname>mvp</varname> plugin uses the proprietary
-          Hauppauge Media MVP interface.  We do not know any user of
-          this plugin, and we do not know if it actually works.
-        </para>
-      </section>
-
       <section>
         <title><varname>httpd</varname></title>