[FIX] payment: prevent deletion of payment acquirer if it has external reference

"ValueError: External ID not found in the system: payment.payment_acquirer_
stripe" is generated because the user deleted the Stripe payment acquirer
record and its corresponding model tried to access the record of it.

Steps to produce the error:
1. Install e-commerce
2. Install install 'Stripe Payment Acquirer ' module
2. delete the Stripe payment from payment acquirer
3. Go to the e-commerce dashoboard
4. Click Set Payment
5. select Credit card (via Stripe)
6. enter any secret and Publishable key
7. click apply

This commit solves the above issue by preventing the deletion
of the payment acquirer if it has a external reference.

sentry-4041178833

closes odoo/odoo#119140

Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>