Don't remove outputs during page flip

Summary:
To do so leaves a dangling pointer on our pageFlipHandler

BUG: 396272

Test Plan: Wobbled a window whilst plugging a monitor in and out

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: anthonyfieroni, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14210