Revise message deletion from SMTP queue

[link2xt]

Currently message sending is implicitly cancelled by removing the message from the database or trashing it:

deltachat-core-rust/src/smtp.rs

Lines 568 to 570 in f930576

	// If there is a msg-id and it does not exist in the db, cancel sending. this happens if
	// delete_msgs() was called before the generated mime was sent out.
	if !message::exists(context, msg_id)

This logic is difficult to test with Rust tests as you need to run actual SMTP sending code for that and does not allow to delete a message without cancelling sending as reported in #2363

This issue is a replacement for #2363 and #4387. #3685 is related.

To close this issue, we need to:

• Remove the code from SMTP sending code that checks if the message is in the msgs table.
• Explicitly delete the message from SMTP queue when the user calls delete_msgs and tests this with a Rust test (fix: Delete messages from SMTP queue only on user demand (#4579) #4615)
• Test with a Rust test in ephemeral.rs that when outgoing message expires, it is deleted from the chat but can still be extracted from SMTP queue on Alice side, received on Bob side and result in a displayed message.

[hpk42]

Just for the record, i just got another accident: two messages sent and deleted while.offline were sent when I came online 10 minutes later. Manual deletion basically doesn't work. This is 1.40.0 android. I reported this issue early may already btw. As #4387

[link2xt]

The fix #4615 is merged into master and I also cherry-picked it into stable as ba2c365