head: remove flattr id

If I recall correctly the service is discontinued

_includes/head.html

@@ -26,7 +26,6 @@
  <meta name="og:description" content="{% if page.excerpt %}{{ page.excerpt | strip_html | strip_newlines | truncate: 160 }}{% else %}{{ site.description | nstrip_html | strip_newlines }}{% endif %}" />
  <meta name="og:site_name" content="{{site.title}}" />
  <meta name="og:image" content="{{site.icon}}"/>
- <meta name="flattr:id" content="7j1ne6">
  <!-- https://indieweb.org/How_to_set_up_web_sign-in_on_your_own_domain#How_to_setup_RelMeAuth - regardless of antipattern, the things I do for verified checkboxes at Mastodon... -->
  <link rel="me" type="application/activity+json" href="https://sauna.social/users/aminda" />
  <link rel="me" type="application/activity+json" href="https://mastodon.pirateparty.be/users/MikaelaPPFI" />