fix: lint unhappy with unused import (#1412)

frontend/app_flowy/packages/appflowy_popover/lib/src/popover.dart

@@ -1,4 +1,3 @@
-import 'dart:async';
 import 'package:appflowy_popover/src/layout.dart';
 import 'package:flutter/material.dart';
 import 'mask.dart';