[FIX] auth_oauth: remove redirect if there is only 1 provider
[FIX] auth_oauth: remove redirect if there is only 1 provider

Forcing the redirection for 1 provider but not for 2 makes no sense.
Was done at 311f041f for the reset password, applied to signup too.

Closes #15032
dfang/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo