Remove icon set credits

OpenPGP-Keychain/src/main/res/raw-de/help_about.html

@@ -37,8 +37,6 @@
 <a href="https://github.com/dschuermann/html-textview">HtmlTextView</a> (Apache Lizenz v2)</li>
 <li>
 <a href="https://github.com/johnkil/Android-AppMsg">Android AppMsg Bibliothek</a> (Apache Lizenz v2)</li>
-<li>Icons von <a href="http://rrze-icon-set.berlios.de/">RRZE Icon Set</a> (Creative Commons Attribution Share-Alike Lizenz 3.0)</li>
-<li>Icons von <a href="http://tango.freedesktop.org/">Tango Icon Set</a> (Public Domain)</li>
 </ul>
 </body>
 </html>