[slidingpopups] Only remove WindowClosedGrabRole if the effect owns the grab

Summary:
The effect unset the WindowClosedGrabRole unconditionally if it does not
manage the window. This results in any grab set by other effects to break.

BUG: 376609
FIXED-IN: 5.9.5

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D5225