Drop changefreq from sitemap.xml.

Closes #3516.

(Should this pull request also update the [release notes](https://github.com/mkdocs/mkdocs/blob/master/docs/about/release-notes.md) or are we waiting until we push a release before we collate those?)

mkdocs/templates/sitemap.xml

@@ -5,7 +5,6 @@
     <url>
          <loc>{% if file.page.canonical_url %}{{ file.page.canonical_url|e }}{% else %}{{ file.page.abs_url|e }}{% endif %}</loc>
          {% if file.page.update_date %}<lastmod>{{file.page.update_date}}</lastmod>{% endif %}
-         <changefreq>daily</changefreq>
     </url>
     {%- endif -%}
 {% endfor %}