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