In showproduct.php there are 6 lines like this you would need to remove to remove the supersize links. You can also elect not to save a supersize image in admin options. Take note the line above is one such example your other lines would be like $file2thumb etc.
Deleting $file1thumb .= "<br /><a href=\"$largeurl\" target=\"_blank\">supersize</a>";
works fine but the moment I deleted the 'same' code for thumb2, thumb3, all I get is a blank page when I view showproduct.php in a browser.