[idn] Canonicalization requirements [21] and [22]

[Paul Hoffman / IMC <phoffman@imc.org>]

[21] and [22] aren't requirements. They were lead-in text for the 
section before the requirements got numbered. Although the other 
sections don't have lead-in text, the text is good and should stay, 
but the numbers removed.

--Paul Hoffman, Director
--Internet Mail Consortium