libweston: fix output destroy signal list corruption

This issue was likely introduced by "libweston: add
weston_view_set_output()" which forgot to ensure the output destroy
listener is removed when weston_view is destroyed, leading to freed
memory being left into the list.

This was quite easy to trigger by opening and closing an application
window a few times, leading various memory corruption symptoms.

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