Delete the password for the username of the service.

def delete_password(service, username):
        """Delete the password for the username of the service.
        """
        try:
            # set up the call for security.
            call = subprocess.Popen(['security',
                                     'delete-generic-password',
                                     '-a',
                                     username,
                                     '-s',
                                     service],
                                    stderr=subprocess.PIPE,
                                    stdout=subprocess.PIPE)
            _, _ = call.communicate()
            code = call.returncode
            # check return code.
            if code is not 0:
                raise PasswordSetError('Can\'t delete password in keychain')
        except:
            raise PasswordSetError("Can't delete password in keychain")