Actually disable RT::Authen::ExternalAuth while importing users from LDAP
The original code from 56d0d416 only stands a chance at working on RT 3.6. RT 3.8 and 4.0 use the modern config handling and call RT->Config->Get('Plugins') to load plugins. Get() doesn't read values from the RT:: namespace; it uses a secondary store which we never affected. By calling Set() ourselves we ensure the config is properly updated and seen by the subsequent RT::Init() call.