Remove the DataMatrix code, just the QR code is enough

Summary:
QR codes are much more widely used (and therefore recognized) and are more
compact, so the DataMatrix one is just taking up more space and makes this
look ugly.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20339