remove superfluous menupopup from get message button, no bug, r+sr=Neil over IRC

mailnews/base/resources/content/mailWindowOverlay.xul

@@ -1722,7 +1722,6 @@
                   </menupopup>
               </rule>
           </template>
-          <menupopup/>
       </toolbarbutton>
       <toolbarbutton id="button-newmsg" class="toolbarbutton-1" label="&newMsgButton.label;" tooltiptext="&newMsgButton.tooltip;" oncommand="MsgNewMessage(event)"/>
       <toolbarseparator class="toolbarseparator-primary"/>