horizon does not uses django-registration

It was removed from the requirements since long, by
a47aa6fdc33ee29be886b45236e276a42a70b4df.

Just the files/rpms/horizon referencing it.

Change-Id: Ic1d413855e80a6f64d9d255f2d3a41f0925332c5
diff --git a/files/rpms/horizon b/files/rpms/horizon
index 2dd24e0..38d349d 100644
--- a/files/rpms/horizon
+++ b/files/rpms/horizon
@@ -1,5 +1,4 @@
 Django
-django-registration
 gcc
 httpd # NOPRIME
 mod_wsgi  # NOPRIME