PanelShadow: Don't remove shadows from destroyed windows

Summary:
If the window's platform surface was destroyed, there is also
no shadow to be removed.
Both X and Wayland code will try to create the platform surface
again, and it will lead to crash in other code that doesn't expect
platform surface to be created when the containment is being deleted.

This fixes crash in PanelConfigView::event when removing panels.

Reviewers: graesslin, Plasma

Reviewed By: graesslin, Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D951