Simplify ticket retrieval from AP-REQs

After krb5_rd_req_decoded or krb5_rd_req_decoded_anyflag, the ticket
(with enc_part2 if we could decrypt it) is accessible via
request->ticket; there is no need to copy it.  Stop using the ticket
parameter of those functions.  Where we need to save the ticket beyond
the lifetime of the krb5_ap_req, steal the pointer before freeing the
request.