libweston: Add paint node destruction into weston_layer_entry_remove()

This prevents a potential crash where users of
weston_layer_entry_insert/layer_entry_remove() would see when moving
views into a NULL layer (effectively unmapping the surface/view).

Users that have migrated to the weston_view_move_to_layer() are immune
to this issue because that takes care of paint node destruction.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>