Remove unnecessary conversion to QList

Summary: chart xy data is converted from std vectore to qlist and then back without apparent reason

Test Plan: chart behaviour tested manually

Reviewers: O3 MITK Reviewer Group II, floca, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Subscribers: kislinsk

Maniphest Tasks: T27167

Differential Revision: https://phabricator.mitk.org/D261