templates: Fix duplicate inclusion of portico CSS.

Apparently, essentially every one of our landing pages extending
portico.html had two copies of portico.css included in their head
section; one from porticocustomhead (or the super of customhead) and
the other directly included.

Clean this up by removing all these duplicate inclusions of the
portico stylesheet.

templates/zilencer/billing.html

@@ -2,10 +2,8 @@
 
 {% block customhead %}
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
-{% stylesheet 'portico' %}
 {% stylesheet 'landing-page' %}
 {{ render_bundle('landing-page') }}
-
 {% endblock %}
 
 {% block portico_content %}