[FIX] base: creation of portal user

When creating a portal user from the User & Companies menu in the
Settings.

Before this commit, the created user had both groups' portal user and
internal user, which generated an error in the display of the user
accesses and rights. This also occurred if we manually add a portal
user to the internal user group.

Now, when creating a portal user, the user only has this group. Also,
if we manually add a portal user to the internal user group, an error
is raised to inform that only one user type is allowed.

OPW-1929367

closes odoo/odoo#30958

Signed-off-by: "Lucas Perais (lpe)" <lpe@odoo.com>