Merge together core subsystems core_hub, core_register and core_publish

core_component::fetch_subsystems() :

            'hub'         => null,

            'publish'     => $CFG->dirroot.'/course/publish',

            'register'    => $CFG->dirroot.'/'.$CFG->admin.'/registration', // Broken badly if $CFG->admin changed.

lib/classes/hub/, lang/en/hub.php - present, contain classes and strings for all three subsystems
admin/registration/classes/ and lang/en/register.php - created during GDPR work for privacy API implementations
course/publish/classes/, lang/en/publish.php - do not exist but would need to be created for privacy API implementations otherwise, see MDL-62136

Basically, there is no reason for components core_publish and core_reigster to exist