bump Qt version to 5.11 and remove unrequired code

Summary:
Bump the minimum Qt version to 5.11.
With D21943, Elisa will effictively depend on 5.11 anyways.
Additionally, Qt 5.10 is now untested by CI.
Bumping also allows to remove a local copy of the
model tester and gives a nice cleanup.

Reviewers: ngraham, mgallien

Reviewed By: ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D22741