Revert MDL-64383

This is terrible fix for something which was not a bug.

Learning Technologists supporting the Moodle platform used the lack of requirement on the username to be able to convert Active Directory/ LDAP Accounts to Manual and back to Active Directory / LDAP.

With this fix, there is no way for Site Admins to deal with emergency cases where for exams making an account manual with a set password is used in the time sensitive cases where students have locked themselves out of Moodle. While the switch from ldap to manual is possible. The switch back from manual to ldap is only possible if one uses the console to remove the disabled="disabled" tag on the username field.

This is also a key way for Site Admins to handle name changes where usernames include  the user's last name involved in the name change so an account can be changed to the new username as manual & then switched back to being an LDAP account.