[potd dataengine] Remove unused X-KDE-PlasmaPoTDProvider-SuffixType

Summary:
Not used by code at least since this dataengine was moved to
kdeplasma-addons in 2009 in 627cf568

Test Plan: All providers still work

Reviewers: xuetianweng, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11952