Remove unused portfolio route

The activity was moved into the admin page, so this separate route is
not necessary anymore.

templates/portfolios/activity/index.html

@@ -1,11 +0,0 @@
-{% extends "portfolios/base.html" %}
-{% from "components/pagination.html" import Pagination %}
-
-{% set secondary_breadcrumb = "navigation.portfolio_navigation.breadcrumbs.admin" | translate %}
-
-{% block portfolio_content %}
-  <div v-cloak>
-    {% include "fragments/audit_events_log.html" %}
-    {{ Pagination(audit_events, 'portfolios.portfolio_activity', portfolio_id=portfolio_id) }}
-  </div>
-{% endblock %}