Do you really need the local account? I would suggest to use it only to make your domain account a wiki admin and to stop using it. There is nothing in the code as far as I can see anyway to stop that message.
However if it really bothers you and dont mind performing a dodgy hack, I guess you could go into the LdapAuth extension directory and modify the file found at src/Groups/LdapGroupSync.php and find the function populateGroups and remove the lines after the if statement that says if (null === $domain) all the way past final brace under throw new MappingException, Would not recommend doing this however.