[server] Don't destroy SlideInterface when parent SurfaceInterface is destroyed

Summary:
Destroying the SlideInterface on the server side before the client has a
chance to cleanup results in a protocol error:
wl_display@1: error 0: invalid object 7

Which would terminate the client. If we would not destroy the resource,
but only delete the SlideInterface it could result in heap-use-after-free.

So just don't do anything, the client needs to cleanup which will result
in the SlideInterface being deleted.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1714