Remove `vg-request-with-auth` from IMAP after processing

[link2xt]

Alice currently does not remove the message from IMAP for some reason stated in the comment:
https://github.com/deltachat/deltachat-core-rust/blob/e7a29f0aa76d0c95f5facfed7030bb0a9651afa4/src/securejoin.rs#L473

In multi-device case this may result in situation when Bob join the group, then leaves it, then second Alice device comes online and processes vg-request-with-auth again and adds Bob back.

I don't see why Autocrypt header is needed by the second device, Alice should see self-sent "member added" message and take gossiped Bob's key from there.

This is a follow-up to #5356

[iequidoo]

I don't see why Autocrypt header is needed by the second device

Probably because it's a common code for the "verified groups" and "verified contacts" setup protocols. Yes, it seems that vg-request-with-auth may be safely IMAP-removed, but we should make sure that gossiping happens even in case of two-member groups and add a test for that.