Remove user ID token (if any) upon USERDATA_REAUTH in MslControl.cleanupContext()

[wmiaw]

Since USERDATA_REAUTH may be returned in some cases when a user ID token is being used (when the responder wishes to force a user to provide new user authentication data) the MslControl.cleanupContext() function should remove the user ID token associated with the user if one exists.

Double-check the Wiki documentation to make sure this change in behavior is documented.