Clean up field value when is not used, this because is only show when some conditions applies, if not we need to clear it to avoid having wrong data on taxes. We want to behave as an onchange, to se change the field to a compute one, in this case stored and also can be edit by the user closes odoo/odoo#156808 Signed-off-by: Josse Colpaert <jco@odoo.com>