Summary:
Resetting timeline while animating a window causes some visual "glitches",
see videos below. Even though we can't use `QTimer::toggleDirection`,
that's safe to delete code which resets timeline because `prePaintScreen`
figures out correct direction of the timeline.

Before

{F5809835, layout=center, size=full}

After

{F5809850, layout=center, size=full}

Test Plan:
* Set duration to 5000
* Furiously click an icon in the task manager

Reviewers: #kwin, davidedmundson

Reviewed By: davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D12253