[FIX] account: do not allow a user to have both B2B and B2C groups

We base ourselves on commit f206714a, which added a check on user groups.
The tax-included and tax excluded displays are meant to be exclusive,
but wily users can add themselves to both groups using the res_users view
in debug mode.

opw 1950833

closes odoo/odoo#32200

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>