<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def delete_credential(self, identifier, credential_id=None):
 """Delete the object storage credential. :param int id: The object storage account identifier. :param int credential_id: The credential id to be deleted. """

        credential = {
            'id': credential_id
        }

        return self.client.call('SoftLayer_Network_Storage_Hub_Cleversafe_Account', 'credentialDelete',
                                credential, id=identifier)