remove unused "multithread filter" menu item

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

src/wx/xrc/MainMenu.xrc

@@ -330,10 +330,12 @@
           <label>_Bilinear filter</label>
           <checkable>1</checkable>
         </object>
+<!--
         <object class="wxMenuItem" name="Multithread">
           <label>_Multithread filter</label>
           <checkable>1</checkable>
         </object>
+-->
         <object class="separator"/>
         <object class="wxMenuItem" name="KeepOnTop">
           <label>_Keep window on top</label>