Bug 25360: (follow-up) Remove the https FIXME in Auth.pm

Bug 25360: (follow-up) Remove the https FIXME in Auth.pm

The FIXME is no longer valid since we fixed the X-Forwarded headers
for Plack. And since we do not even use using_https anymore in
the templates (see bug 21094).

Test plan:
Run Auth.t
Git grep for using_https

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>