Remove footer of tails-support-private@

The footer of emails sent by tails-support-private@ still reads:

« Our help desk is helping hundreds of Tails users each month. Each user request costs us $6 on average to process. If you find it useful, please consider donating to Tails so we can help more people like you: https://tails.boum.org/donate?r=help »

That's not true anymore since we usually limit ourselves to reading these email. Also the figure is probably very wrong and I never saw a lot of donations coming from that.

Could you please remove it from the Schleuder configuration? Thanks!