compositor: Remove layer_entry_remove/layer_entry_insert

There are no longer users of these two function. With the
weston_view_move_to_layer() helper being capable of doing this it is time
to retire these entirely to avoid users using them.

Release notes should mention that migrating to the newer helper will be
required when bumping to Weston 14.

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