remove future (#6065)

superset/models/annotations.py

@@ -1,11 +1,6 @@
 # -*- coding: utf-8 -*-
 # pylint: disable=C,R,W
 """a collection of Annotation-related models"""
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
-from __future__ import unicode_literals
-
 from flask_appbuilder import Model
 from sqlalchemy import (
     Column, DateTime, ForeignKey, Index, Integer, String, Text,