Fix harmless warning about unused variable in wxMSW wxMenu

A variable was unused in wxDEBUG_LEVEL==0 build and assigning to it resulted
in a warning. Fix this by not defining this variable at all, which also makes
the check more clear.

src/msw/menu.cpp

@@ -738,11 +738,13 @@ wxMenuItem *wxMenu::DoRemove(wxMenuItem *item)
     // Update indices of radio groups.
     if ( m_radioData )
     {
-        bool inExistingGroup = m_radioData->UpdateOnRemoveItem(pos);
-
-        wxASSERT_MSG( !inExistingGroup || item->GetKind() == wxITEM_RADIO,
+        if ( m_radioData->UpdateOnRemoveItem(pos) )
+        {
+            wxASSERT_MSG( item->GetKind() == wxITEM_RADIO,
                           wxT("Removing non radio button from radio group?") );
         }
+        //else: item being removed is not in a radio group
+    }
 
     // remove the item from the menu
     if ( !::RemoveMenu(GetHmenu(), (UINT)pos, MF_BYPOSITION) )