libweston: Remove signals from the list during de-init

During de-init ensure removal of added signals from list. Otherwise
a dongling pointer is left behind which will affect other plugins.

Signed-off-by: Harsha M M <harsha.manjulamallikarjun@in.bosch.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>