attic: uninit

Attic is currently broken on master.
The Attic git repo has not been updated since May 2015.
Arch Linux also does not have an Attic package anymore.
Borg should be able to read Attic backups using
http://borgbackup.readthedocs.io/en/stable/usage/upgrade.html#attic-and-borg-0-xx-to-borg-1-x.

nixos/doc/manual/release-notes/rl-1803.xml

@@ -231,6 +231,14 @@ following incompatible changes:</para>
       <literal>Option "XkbRules" "base"</literal> may result in broken keyboard layout.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      The <literal>attic</literal> package was removed. A maintained fork called
+      <link xlink:href="https://www.borgbackup.org/">Borg</link> should be used instead.
+      Migration instructions can be found
+      <link xlink:href="http://borgbackup.readthedocs.io/en/stable/usage/upgrade.html#attic-and-borg-0-xx-to-borg-1-x">here</link>.
+    </para>
+  </listitem>
 </itemizedlist>
 
 </section>