OK, it seems like mint_idea doesn't have a class "asterisk" defined in the stylesheet. But the class is in the question.pstpl:
<span class="asterisk">{QUESTION_MANDATORY}</span>
So what you do is to create a class in the template.css and hide this class:
.asterisk {
display:none;
}
Or you could just remove the whole code above, so the asterisk doesn't show anymore. However, just to inform you: the asterisk is there to alert participants about mandatory questions. If you hide/delete it, there is no sign of the question being mandadory.