With this commit, Selection fields with `required=True` which are extended via `selection_add` are given proper ondelete policies to ensure the cleanup of records containing these extended options during uninstall of the extending module. This commit also cleans up leftover uninstall hooks that were being used to handle the same set of problems prior to the ondelete mechanism being implemented for Selection fields. closes odoo/odoo#46325 Related: odoo/enterprise#9117 Signed-off-by: Raphael Collet (rco) <rco@openerp.com>