Follow-up of 13a45447

Where we fixed the portal *list* of opportunities but did not notice that the
associated *form* also had the same issue.

To recap the issue:

When you remove the 'company_id' on a crm.lead, you also remove the computed
'company_currency'.
When trying to view this kind of leads in the website_crm_partner_assign portal
form page, it would raise an error while trying to display the planned_revenue
in the missing currency.

Now, we display the number without any currency sign, which is a "best effort"
solution, just the same as on the crm.lead form view.
So it will look like "9000 at 47%" instead of "$9000 at 47%".

Task-2444539

closes odoo/odoo#64933

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>