[server] Remove sub-surface from parent when it's surface is destroyed
Summary:
From spec:
If the wl_surface associated with the wl_subsurface is destroyed,
the wl_subsurface object becomes inert. Note, that destroying either
object takes effect immediately.
Reviewers: Plasma
Subscribers: plasma-devel
Projects: Plasma
Differential Revision: https://phabricator.kde.org/D1211