<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_signing_cert(self, cert_id, user_name=None):
 """ Delete a signing certificate associated with a user. If the user_name is not specified, it is determined implicitly based on the AWS Access Key ID used to sign the request. :type user_name: string :param user_name: The username of the user :type cert_id: string :param cert_id: The ID of the certificate. """

        params = {'CertificateId' : cert_id}
        if user_name:
            params['UserName'] = user_name
        return self.get_response('DeleteSigningCertificate', params)