[FIX] pos_mercury: error when adding vantiv payment

Since 8fb53c53, Order model is no longer
a backbone model therefore, we can no longer call the "trigger" method
on it. Normally, the old "trigger" call means we want to rerender the
screen and persist the new order information in the local storage.
The order object is already setup to do those mentioned (rerendering
and saving to local storage) when it's mutated. Therefore, we can just
simply remove the "order.trigger" and "this.render" calls as proposed
in this commit.

closes odoo/odoo#116642

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>