Use the default `url` tag instead

`url` change:
https://docs.djangoproject.com/en/1.8/internals/deprecation/#deprecation-removed-in-1-5
See notes for the url tag change

Partially-Implements: blueprint drop-dj17

Change-Id: Ifa20dbe3b617fa05214c61c4e392a422b2270807

horizon/templates/auth/_login_form.html

@@ -1,5 +1,4 @@
 {% load i18n %}
-{% load url from future %}
 
 {% block pre_login %}
 <form id="" class="ng-pristine ng-valid ng-scope"