refactor: remove ConfiguredAddr

[link2xt]

Closes #8572

[link2xt]

@pbuzdin please create a separate account for your bot and don't let it post any comments in any chatmail or deltachat organization repositories. There are some community standards at https://delta.chat/en/community-standards, but even if they were not written down, it should be obvious that it is not great to run a bot that posts comments here pretending to be a human and then note that "a first-class preferred-transport concept has been requested there (comment 6090501441)" in pbuzdin/velta#114 (comment)

[pbuzdin]

That wasnt a bot comment. I wrote it myself because I'm having trouble using Deltachat in my region without manually selecting a relay for sending.

[r10s]

our idea for situation "one relay cannot send" is to improve sending failover. or, in the future, replacing that relay automatically.

adding more options, that most ppl would not understand, is often not a solution from our view. a configured_addr or used_for_sending were always meant as temporary things since we introduced multi relay. not having tons of options is also easier for us to debug and to get the model.

nerds detecting smth is wrong with a relay, by some log or so, can remove it.

as a general rule of thumb, we avoid options and beforehand questions whereever we can.

btw: the screenshots of velta look great!