Fix RSS template (#969)

Remove a link tag mistakenly imported from Atom XML namespace. The tag
was used to specify the link to the feed itself which is not supported
by RSS 2.0:
https://cyber.harvard.edu/rss/rss.html

Fixes #967

components/templates/src/builtins/rss.xml

@@ -1,12 +1,11 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<rss xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">
+<rss version="2.0">
     <channel>
         <title>{{ config.title }}</title>
         <link>{{ config.base_url | escape_xml | safe }}</link>
         <description>{{ config.description }}</description>
         <generator>Zola</generator>
         <language>{{ config.default_language }}</language>
-        <atom:link href="{{ feed_url | safe }}" rel="self" type="application/rss+xml"/>
         <lastBuildDate>{{ last_build_date | date(format="%a, %d %b %Y %H:%M:%S %z") }}</lastBuildDate>
         {% for page in pages %}
             <item>