[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