More import removals

Following the series of commits removing deprecated features in
Django 1.7, here are some more unneeded imports removed and other
minor cleanups.

django/test/testcases.py

@@ -15,7 +15,6 @@ except ImportError:     # Python 2
 import select
 import socket
 import threading
-import warnings
 
 from django.conf import settings
 from django.contrib.staticfiles.handlers import StaticFilesHandler