Before this PR, when a user with current main sends a large message to a
user with an old Delta Chat (before #7431), the text will be duplicated:
One message will arrive with only the text, and one message with
attachment+text.
This PR changes this - there will be one message with only the text, and
one message with only the attachment.
If we want to guard against lost pre-messages, then we can revert this
PR in a few months, though I'm not sure that's necessary - it's unlikely
that the small pre-message gets lost but the big post-message gets
through.
