it will be in file templates/flexible_darkmart/html/com_virtuemart/productdetails/default.php ~ line 632
you would just surround the <li> in a conditional statement
e.g.
<?php if (!empty($this->product->product_desc)) { ?>
<li class="selected"><a href="javascript:void(0)" rel="#desc"><span><?php echo JText::_('COM_VIRTUEMART_PRODUCT_DESC') ?></span></a></li>
<?php } ?>
and also the div on line 684
<?php if (!empty($this->product->product_desc)) { ?>
<div id="desc" class="tabcontent">
<div class="product-description">
<?php echo $this->product->product_desc; ?>
</div>
</div>
<?php } ?>
but your going to have to play with the ids and style="display:none;" etc to see what haddens
these would have to be conditional as well
Cheers