[FIX] website_crm_partner_assign: fix opportunities portal page without currency

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
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 2416841

closes odoo/odoo#63552

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