b=328070 opt-in UI for data collection r=Jan.Varga --pete

extensions/metrics/content/prefs.xul

@@ -24,9 +24,4 @@
                 persist="checked" class="cb" />
     </vbox>
   </prefpane>
-
-  <hbox flex="1" pack="end">
-    <button label="&prefs.button.close;" oncommand="close();" />
-  </hbox>
-
 </prefwindow>