do you really want to do this, and not honor the creators of the theme and the software, by simply leaving this link to wordpress in your site?
Nonsense. TwentyTen is released under GPL. The end user has every right to modify the Theme in any way whatsoever.
(The note in question actually refers to removing the HTML-tag markup in the “footer.php” template file. Changing that markup can/will have adverse consequences on the rendering of the site, if such changes result in invalid markup.)
But I’d do it as a Child Theme, to which is added the modified “footer.php” template file; otherwise, the change will get over-written when TwentyTen is updated in the future.