Remove mention of unspecified key binding methods

[bc-pi]

Remove mention of unspecified key binding methods and the Enveloping SD-JWTs section (issue #383)

Editor's copy preview of this PR: https://drafts.oauth.net/oauth-selective-disclosure-jwt/remove-mention-of-unspecified-key-binding-methods/draft-ietf-oauth-selective-disclosure-jwt.html

[danielfett]

Thank you, approved with one nit.

[Sakurann]

Suggested change

	3. If Key Binding is required:
	3. If Key Binding JWT is required:

if Key Binding is required, but can be provided using means other than KB JWT, the current 3.1 is too strict

[Sakurann]

noting that this change solidifies that to be compliant to the sd-jwt spec, one must use kb jwt for key binding, more than merely removing a note that key binding can be provided using means other than kb jwt.