[server] Don't destroy ShadowInterface when SurfaceInterface gets destroyed

Summary:
Unlike the other cases this one is not as dangerous as the shadow
protocol doesn't have a destructor request (yet).

Once that is added the problem would be the same: destroying the
ShadowInterface when the parent SurfaceInterface gets destroyed would
result in a protocol error on client side.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1711