Drop Flattr since the link is 404 and the website is paywalled

Ref: #99 (comment)

pmaweb/templates/donate.html

@@ -69,14 +69,6 @@ <h3>Stock donations</h3>
 for instructions on how to initiate the transfer.
 </p>
 
-<h3>Flattr</h3>
-
-<p>
-Alternatively you can appreciate our work using 
-<a href="https://flattr.com/@sfconservancy/domain/phpmyadmin.net">Flattr</a>. Flattr is a
-microdonation system allowing users to easily appreciate others.
-</p>
-
 <p>
 Donations will be used for the promotion of phpMyAdmin, mostly by allowing team members to visit various conferences. Thanks.
 </p>