[FIX] point_of_sale: Prevent multiple cash payment method in PoS

Current behavior:
When creating a PoS config, it is possible to have multiple cash payment
method. Wich is not supported by the PoS.

opw-3030125

closes odoo/odoo#109236

X-original-commit: f9da3590
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>