KSMTP: remove dependency on KMime, by removing setMessage().

Summary:
Update API DOX and unittest to use setData(QByteArray).
In practice, kmailtransport/plugins/smtp is already using that directly.

Test Plan: Compiles, unittest passes, all of kde/pim still compiles.

Reviewers: dvratil, vkrause

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12432