Don't set legacy smtp_certificate_checks config

[Hocuri]

At

deltachat-ios/deltachat-ios/DC/DcContext.swift

Line 712 in 5caaa9d

setConfig("smtp_certificate_checks", "\(newValue)")

, you are setting the legacy "smtp_certificate_checks" config. It's been deprecated since chatmail/core#5854, and I would like to remove it entirely in chatmail/core#6803

So, please remove the line I linked above. cc @r10s @zeitschlag