Remove unneeded __futures__ imports

No longer needed with Python 2 support dropped.

evennia/typeclasses/migrations/0011_auto_20190128_1820.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.16 on 2019-01-28 18:20
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import evennia.utils.picklefield

evennia/typeclasses/migrations/0012_attrs_to_picklev4_may_be_slow.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.16 on 2019-01-30 20:56
-from __future__ import unicode_literals
 
 from copy import deepcopy
 from base64 import b64encode, b64decode