Drop VirtualBox support
Tails' VirtualBox support is really bad (e.g. #19324 (closed), #18781 (closed)) due to lack of guest additions (#18728 (closed)).
The manual test we do each release feels is really useless. In fact, when the test was originally added it was specifically about the guest additions, presumably because they kept breaking. What do we get from testing Tails in VirtualBox without guest additions? If we get anything I doubt it is worth it, and that there are more important things we do not manually test.
I don't think we in good faith can continue to advertise VirtualBox support to users in this poor state. We we're already considering completely dropping VirtualBox support back in Oct 2021 when we stopped compiling the guest additions, but we had some optimistic ideas that didn't pan out (#18643 (comment 179126), #18666 (closed)), and here we are now, over two years later, still pretending things are good or are going to get better. IMHO it is high time to drop it, and even if someone comes up with another optimistic plan I don't think it should prevent us from dropping it, instead let them restore the docs/tests when they actually land the fix in Tails.