[REM] core: remove deprecated v7 onchange support

Old API support has been dropped as of 11.0, and the old onchange system
should have been discontinued along with it.

New-API onchange mechanism is automatically triggered without requiring
any extra markup in view declarations, as explained in the
documentation:
- https://www.odoo.com/documentation/11.0/reference/orm.html#porting-from-the-old-api-to-the-new-api
- https://www.odoo.com/documentation/11.0/reference/orm.html#onchange-updating-ui-on-the-fly