[server] Wrap proxyRemoveSurface in smart pointer

Summary:
proxyRemoteSurface is a fake wayland surface created for the drag and
drop between X and wayland clients.

The lifespan is not guaranteed to outlive the DataDevice.

Kwin closes Xwl before the other wayland client connections so it's
technically dangling for a bit when events could be dispatched.

Probably will never happen, but better to guard and be safe.

Test Plan: Compiles

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22862