Bug 1756359 - Replace use of removeItemAt in FilterListDialog. r=frg

Port the relevant parts of:
* Bug 1459184 - Port bug 1456611: Replace use of insertItemAt and removeItemAt methods (mail part)

suite/mailnews/content/FilterListDialog.js

@@ -447,7 +447,7 @@ function onDeleteFilter()
   for (let index = items.length - 1; index >= 0; --index) {
     let item = items[index];
     gCurrentFilterList.removeFilter(item._filter);
-    gFilterListbox.removeItemAt(gFilterListbox.getIndexOfItem(item));
+    item.remove();
   }
   updateCountBox();