We should use admin_settings_coursecat_select instead of
enrol_database_admin_setting_category as the second class is only
being used by the database enrolment plugin settings and can be
removed after this change.
Includes fixes for minor problems discovered during testing:
* Removed invalid role->fullname sync option, there was never such fields in roles table and the name column is now usually empty.
* Empty category column now means use default category.