remove old flattr button from donations list

The old flattr button no longer works, and seem it hasn't since 2018
https://trisquel.info/en/forum/flattr-donations-link-button-broken-httpstrisquelinfo

Link of flattr: http://flattr.com/thing/399387/Trisquel-GNULinux

So it will be a great idea to remove it on the incoming version.