[FIX] account: unlink account journal

The bank account linked to a journal cannot be systematicaly deleted
with its journal because this bank account can be linked to several
journals.

opw:676374

addons/account/models/account.py

@@ -293,7 +293,11 @@ class AccountJournal(models.Model):
 
     @api.multi
     def unlink(self):
-        bank_accounts = self.mapped('bank_account_id')
+        bank_accounts = self.env['res.partner.bank'].browse()
+        for bank_account in self.mapped('bank_account_id'):
+            accounts = self.search([('bank_account_id', '=', bank_account.id)])
+            if accounts <= self:
+                bank_accounts += bank_account
         ret = super(AccountJournal, self).unlink()
         bank_accounts.unlink()
         return ret