Remove Secure-Join-Fingerprint header (eventually) from vc-contact-confirm

It is not used since version 1.107.0 released on 2023-02-01.

There is a commit in a postponed PR: https://github.com/deltachat/deltachat-core-rust/pull/4345
We just need to cherry-pick https://github.com/deltachat/deltachat-core-rust/pull/4345/commits/4da9e392ca2924f8bb7076d7e0426b8e03bfb571 eventually, probably after 1.42 releases are stabilized.