ive been trying to do something like this today. ive tryed looking through the PHP code an have had a lot of trouble finding where it shows this link. if you want to get rid of it compleatly you can change the language file in:
moodle/lang/en_utf8/forum.php
if you open up that file and change:
$string['subscribe'] = 'Subscribe to this forum'; to $string['subscribe'] = '';
$string['unsubscribe'] = 'Unsubscribe from this forum'; to $string['unsubscribe'] = '';
However if anyone knows a proper way of hideing this im all ears?