Journal entry field is not a mandatory field in customer/supplier payments. It is many2one field account.move object and that is automatically created on Validating the payment. If you are talking about Journal field, then we cannot make that field non-mandatory as this field is a mandatory field in journal entry which is created automatically on Validation.

Mandatory fields we are giving in two ways. Either in python file on field declaration or in xml file. If required=True is given in python file, we have to rewrite the field in our module without giviong required attribute. If it is given in xml file, we have to inherit and change the attribute.

<xpath expr="//field[@name='field_name']" position="attributes">
                    <attribute name="required">0</attribute>
                </xpath>