This PR removes a mention of refresh tokens.

[adeinega]

RFC8693 does allow to get a RT in some cases but does not take / exchange it.

A refresh token will typically not be issued when the exchange is of one temporary credential (the subject_token) for a different temporary credential (the issued token) for use in some other context. A refresh token can be issued in cases where the client of the token exchange needs the ability to access a resource even when the original credential is no longer valid (e.g., user-not-present or offline scenarios where there is no longer any user entertaining an active session with the client). Profiles or deployments of this specification should clearly document the conditions under which a client should expect a refresh token in response to urn:ietf:params:oauth:grant-type:token-exchange grant type requests.

[deansaxe]

lgtm