[FIX] purchase: remove duplicate code

Due to a forward-port, the code updating the vendor_ref has been duplicated
a few lines apart.

Closes odoo/odoo#33334

closes odoo/odoo#33676

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

addons/purchase/models/account_invoice.py

@@ -103,11 +103,6 @@ class AccountInvoice(models.Model):
             #as there's no invoice line yet, we keep the currency of the PO
             self.currency_id = self.purchase_id.currency_id
 
-        vendor_ref = self.purchase_id.partner_ref
-        if vendor_ref:
-            self.reference = ", ".join([self.reference, vendor_ref]) if (
-                    self.reference and vendor_ref not in self.reference) else vendor_ref
-
         new_lines = self.env['account.invoice.line']
         for line in self.purchase_id.order_line - self.invoice_line_ids.mapped('purchase_line_id'):
             data = self._prepare_invoice_line_from_po_line(line)