[FIX] account: unset currency_id if equal to company_currency_id

Activate Multicurrency
Create a Journal Entry with default currency (USD)
Save

Error will trigger because of the sql constraint
'check_amount_currency_balance_sign'
The currency_id and company_currency_id are the same while they
shouldn't because company_id should be unset if it is the same as the
company_currency.

Fix to update the error message to let the user know that it's best to
keep the field empty so the company_id would not be set

opw-2169523

closes odoo/odoo#43011

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>