disable donate onion site and remove onion location header

donate's onion site hasn't worked for at least a few years. it wasn't working when i started working on donate 2 years ago. the remnants of the onion site code in the donate middleware uses v2 onion URLs, so it probably hasn't worked for even longer than 2 years.

since we plan on deprecating the current donate site in favor of donate-neo, we're going to deprecate the onion site now so that TBB users aren't misled into thinking the onion site works. those users should still be able to donate through stripe, paypal, btcpay, or through our crypto wallets.

i don't have a proper estimation for this ticket, but it'll probably take a few days to a week at most. the most time consuming part will be finding where the onion site is configured, actually removing it should only take an hour or two.