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
Merge "horizon does not uses django-registration"