desktop-shell: remove surface listener when surface is destroyed

desktop-shell: remove surface listener when surface is destroyed

There may be race condition between destroying surface and destroying
output. If handle_output_destroy() is called after surface is destroyed,
illegal memory access occurs when surface destroy signals is
unregistered from the panel/background. This patch fixes this issue and
removes unnecessary initialization for panel surface listener.

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>