[effects/magiclamp] Don't reset timeline

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

After

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