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; ?> &lsaquo; <?php bloginfo('name') ?>  &#8212; WordPress</title>