Remove unused multipart/mixed when there's no attachments

[tcitworld]

The envelope isn't needed when we're just doing multipart/alternative with just text & html.

Also it triggers rspamd CTYPE_MIXED_BOGUS criteria (1 point by default)

[creusefond]

We encountered a massive prod issue due to this multipart/mixed added at the end of these emails. Some email providers considered our emails as spam due to this, and about ~20% of our emails were affected during the month that was necessary to pinpoint this issue. Would it be possible to merge this PR ?

[arjan]

:+1:

This explains why thunderbird shows a paperclip icon next to the message, but when clicking it, the paperclip disappears. Please merge!