Remove duplicate content_rating tag

Fixes the appstreamtest, i.e. stops appstreamcli validate from
complaining.

org.kde.kcharselect.appdata.xml

@@ -274,7 +274,6 @@
       <image>https://www.kde.org/images/screenshots/kcharselect.png</image>
     </screenshot>
   </screenshots>
-  <content_rating type="oars-1.1"/>
   <project_group>KDE</project_group>
   <provides>
     <binary>kcharselect</binary>