[effects/slidingpopups] Do not add full window repaint in windowDeleted slot

Summary:
Full window repaints issued by the Sliding Popups effect can hide bugs in
other effects that animate the disappearing of windows (e.g. Scale).

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15822