desktop-shell: fix output destroy signal list corruption

This issue was introduced by "desktop-shell: detect stale shell surface
outputs" which forgot to remove the output destroy listener when
shell_surface is destroyed, leading to memory corruption.

This was fairly easy to trigger by opening and closing an application
window a few times.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>