Merge Interface::MessagePart and MessagePart
Summary:
Interface::MessagePart is neither pure-virtual nor is it ever instantiated
now that legacy handling has been separated out, which removes its reason
to exist. This allows simplifying its usage, as we no longer need to
deal with the distinction.
Reviewers: knauss
Reviewed By: knauss
Subscribers: KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D8068