Remove publishonmoodleorg and shareonmoodleorg

As tsala noticed recently, the core_hub subsystem still provides strings

$string['publishonmoodleorg'] = 'Publish on MOOCH';

$string['shareonmoodleorg'] = 'Upload this course to MOOCH';

that were never actually used. In AMOS, they have been greylisted since Moodle 2.0. I believe it is safe to remove them from all supported STABLE branches as well as from the master branch (even though the policy is to remove strings from master only, this is an exception as the strings were never actually used).