Remove DarksendConfig

[UdjinM6]

Turned out that it's not used now (due to a bug) and probably wasn't used at all since it was added 🙈 We can either fix the bug and adjust the dialog OR remove this stuff completely. I think the later is a better option. Changes are pretty trivial but I'm postponing this till 12.4 just in case.

Kudos to @paulied for discovering this. Somewhat complimentary to #2128.

[nmarley]

utACK

Any plans to bring this back eventually, or just remove for good?

[UdjinM6]

Maybe we could extend intro dialog with Dash-specific settings instead or turn it into some kind of wizard if there is a need for that in short/mid term. In principle, I personally hope to see GUI part to be decoupled, moved to its own repo and then be completely rebuilt (maybe even from scratch and maybe not using Qt). But that's a very long term goal I'd say, so for now I just want to get rid of smth obsolete :)