Edit: You should not do this, back in the time when I answered this, I didn't know better. Check this answer instead.
I personaly don't think you should really do this, and I would leave it there if I were you - it is in fact a way to support WordPress, and don't forget you got it for free. ;)
but, if you really want to do it, go to /wp-admin/admin-header.php
and remove — WordPress from this line:
<title><?php echo $title; ?> ‹ <?php bloginfo('name') ?> — WordPress</title>